Since the time when Duméril made known the
transformation of a number of Axolotls into the
so-called Amblystoma form, this Mexican Amphibian
has been bred in many European aquaria,
chiefly with the view to establish the conditions
under which this transformation occurred, so as to
be enabled to draw further conclusions as to the
true causes of this exceptional and enigmatical
metamorphosis.
Although the Amphibians propagated freely,
the cases in which transformation occurred remained
extremely rare, and it was not once
possible to reply to the main question, viz.
whether this metamorphosis was determined by
external conditions or by purely internal causes;
to say nothing of the possibility of there perhaps
being discoverable certain definite external influences
by means of which the metamorphosis
could have been induced with certainty. But
while these points are undecided all attempted
theoretical interpretations of the phenomenon
must be devoid of a solid basis.
It appeared to me from the first that the history
of this transformation of the Axolotl was of
special theoretical value; indeed I believed that
it might possibly furnish a special case for deciding
the truth of those ground-principles,
according to which the origin of this species is
represented by the two conflicting schools as a
case of transformation or as one of heterogenesis.
I therefore determined to make some experiments
with the Axolotl myself, in the hopes of being
fortunate enough to be able to throw some light
upon the subject.
In the year 1872 Prof. v. Kölliker was so good
as to leave with me five specimens of his Axolotls,
bred in Würzburg, and these furnished a numerous
progeny in the following year. With these I
carried out the idea, the theoretical bearing of
which will be shown subsequently, whether it
would not be possible to force all the larvæ, or
at any rate, the greater majority, to undergo
transformation by exposing them to conditions of
life which made the use of gills difficult, and that
of lungs more easy; in other words, by compelling
them to live partly on land at a certain
stage of life.
During that year indeed I obtained no results,
most of the larvæ perishing before the time for
such an experiment had arrived, and the few
survivors did not undergo transformation, but
lived on to the following spring and then also
died one after the other. Through long absence
from Freiburg, necessitated by other labours, I
had evidently left them without sufficient care
and attention. I was thus led to the conviction,
which was more fully confirmed subsequently,
that no results can be obtained without the
greatest care and attention in rearing, towards
which single object all one’s interest should be
concentrated, and it must not be considered
irksome to have to devote daily for many months
a large amount of time to this experiment. As
it was evident that I could not afford this time
without calling in other aid, I hailed with pleasure
an opportunity of witnessing the experiment performed
by other hands.
A lady living here (Freiburg), Fräulein v.
Chauvin, undertook to rear a number of my larvæ
of the following year which had just hatched, and
in accordance with my idea to make the experiment
of forcibly compelling them to adopt the
Amblystoma form. How completely this was
accomplished will be seen from the following
notes by the lady herself, and it will no less
appear that these results were only obtained by
that care in treatment and delicacy of observation
which she devoted to the experiments.
“I began the experiments on June 12th, 1874,
with five larvæ about eight days old, these being
the only survivors out of twelve. Owing to the
extraordinary delicacy of these creatures, the
quality and temperature of the water, and the
nature and quantity of their food exerts the greatest
influence, especially in early life, and one cannot
be too cautious in their treatment.
“The specimens were kept in a glass globe of
about thirty centimeters in diameter, the temperature
of the water being regulated; as food at
first Daphnids, and afterwards larger aquatic
animals were introduced in large numbers. By
this means all the five larvæ throve excellently.
At the end of June the rudiments of the front
legs appeared in the most vigorous specimens,
and on the 9th of July the hind legs also became
visible. At the end of November I noticed that
one Axolotl remained constantly at the surface of
the water, and this led me to suppose that the
right period had now arrived for effecting the
transformation into Amblystoma. For brevity I
shall designate this as No. I., and the succeeding
specimens by corresponding Roman numerals.
“In order to bring about this metamorphosis,
on December 1st, 1874, No. I. was placed in a
large-sized glass vessel containing earth arranged
in such a manner that, when the vessel was filled
with water, only one portion of the surface of the
earth was entirely covered by the liquid, and the
creature in the course of its frequent perigrinations
was thus more or less exposed to the air. The
water was gradually diminished on the following
days, during which period the first changes made
their appearance in the Amphibian—the gills
commenced to shrivel up, and at the same time
the creature showed a tendency to seek the
shallowest spots. On December 4th, it took
entirely to the land, and concealed itself among
some damp moss which I had placed on a heap
of sand on the highest portion of the earth in the
glass vessel. At this period the first ecdysis
occurred. Within the four days from the 1st to
the 4th of December, a striking change took
place in the external appearance of No. I., the gill-tufts
shrivelled up almost entirely, the dorsal
crest completely disappeared, and the tail, which
had hitherto been broad, became rounded and
similarly formed to that of a land salamander.
The grey-brown colour of the body changed gradually
into a blackish hue; isolated spots, at first
of a dull white, made their appearance and these in
time increased in intensity.
“When the Axolotl left the water on December
4th the gill-clefts were still open, but these closed
gradually, and after about eight days were overgrown
with skin and no longer to be seen.
“Of the other larvæ three appeared at the end
of November (i.e. at the same time when No. I.
came to the surface of the water) to have kept
pace in development with No. I., an indication
that for these also the right period had arrived for
accelerating the developmental processes. They
were therefore submitted to the same treatment as
No. I. No. II. became transformed at the same
time and exactly in the same manner as the latter;
its gill-tufts were complete when it was first placed
in the shallow water, but after four days these had
almost entirely disappeared; in the course of about
ten days after it took to the land, the overgrowth
of skin on the gill-clefts and the complete assumption
of the salamander form occurred. During
this last period the creature took food, but only
when urged to do so.
“In Nos. III. and IV. the development proceeded
more slowly. Neither of these so frequently
sought the shallow spots, nor did they as a rule
remain so long exposed to the air, so that the
greater part of January had expired before they
took entirely to the land. Nevertheless the dessication
of the gill-tufts did not take a longer time
than in Nos. I. and II. as the first ecdysis occurred
as soon as they took to the land.
“No. V. showed still more striking deviations in
its transformation than Nos. III. and IV., but as
this specimen appeared much weaker than the
others from the beginning and was retarded in
growth to a most notable extent, this is by no
means surprising. It took fourteen instead of four
days before the transformation had advanced far
enough to enable it to leave the water. It was
especially interesting to observe the behaviour of
this specimen during this period. Its weak and
delicate constitution evidently made it much more
susceptible to all external influences than the others.
If exposed to the air for too long a time it acquired
a light colour, and when annoyed or alarmed it
emitted a peculiar odour, similar to that of a
salamander. As soon as these phenomena were
observed it was at once placed in deeper water, into
which it immediately plunged and gradually recovered
itself, the gills always becoming again
expanded. The same experiment was repeated
several times and always led to the same
result, from which we may venture to conclude
that by accelerating the transformation
too energetically, the process may come to a
standstill, and even by continued compulsion may
end in death.
“It yet remains to be mentioned with respect to
Axolotl No. V. that this specimen, unlike all the
others, did not emerge from the water at the first
ecdysis, but at the time of the fourth.
“All the Axolotls are now (July, 1875) living,
and are healthy and vigorous, so that with respect
to their state of nourishment there is nothing to
prevent their propagating. Of the first four the
largest is fifteen centim. long; Axolotl No. V.
measures twelve centim.
“The preceding statements appear to demonstrate
the correctness of the views advanced in the
Introduction:—Axolotl larvæ generally but not
always complete their metamorphosis if, in the
first place, they emerge sound from the egg and
are properly fed; and if, in the next place, they are
submitted to the necessary treatment for changing
aquatic into aërial respiration. It is obvious that
this treatment must only be applied very gradually,
and in such a manner as not to overtax the vital
energy of the Amphibian.”
* * * * *
To the foregoing remarks of Fräulein v. Chauvin
I may add that in all five cases the transformation
was complete, and not to be confounded with that
change which occurs more or less in all Axolotls
in the course of time when confined in small glass
vessels. In this last case there frequently appear
changes in the direction of the Amblystoma form
without the latter being actually reached. In the
five adult Axolotls which I possessed for a short
time, and of which two were at least four years old,
the gills were much shrivelled, but the aquatic tail
and dorsal crest were unchanged. The crest may,
however, also disappear, and the tail become
shortened without these changes being due to a
transformation into Amblystoma, as will be shown
further on.
With respect to the duration of the transformation,
this amounted in Axolotls Nos. I. to IV.
altogether to twelve or fourteen days. Of these,
four days were taken by the first changes which
occurred while the creature was still in the water;
the remaining time, to the completion of the metamorphosis,
was passed on land. Duméril gives
the duration of the metamorphosis as sixteen
days.
The following results of the experiments just described
appear to me to be especially noteworthy:—The
five Axolotl larvæ which can alone be taken
into consideration, the others having soon perished,
all experienced metamorphosis, and without an exception
became Amblystomas. Only one of them,
No. I., by persistently swimming at the surface,
as was observed at the end of six months, showed
a decided tendency to undergo metamorphosis
and to adopt aërial respiration. With respect to
this specimen it may therefore be confidently
assumed that it would have taken to the land, and
that metamorphosis would have occurred without
artificial aid, just as was the case in the thirty
specimens which Duméril altogether observed.
Respecting Nos. II., III., and IV., on the other
hand, such a supposition is but little probable.
These three larvæ endeavoured to keep in deep
water and avoided as long as possible the shallow
places which would have enforced them to take
entirely to lung breathing. Metamorphosis thus
occurred more than a month later in these
individuals.
Finally, there can scarcely be any doubt that
No. V. would not have become transformed without
forcible adaptation to an aërial life.
From these results we may venture to conclude
that most Axolotl larvæ change into the Amblystoma
form when, at the age of six to nine months,
they are placed in such shallow water that they are
compelled to respire chiefly by their lungs. The
experiments before us are certainly at present but
very few in number, but such a conclusion cannot
be termed premature if we consider that out of
several hundred Axolotls (the exact number is not
given) Duméril obtained only about thirty Amblystomas,
while v. Kölliker bred only one Amblystoma
out of a hundred Axolotls.
It now only remains questionable whether each
larva could have been forced to undergo metamorphosis,
but this could only be decided by new
experiments. It was originally my intention to
have delayed the publication of the experiments
till Fräulein v. Chauvin had repeated them in larger
numbers, but as my Axolotls have not bred this
year (1875) I must abandon my scheme, and this
can be done the more readily because, for the
theoretical consideration of the facts, it is immaterial
whether all or only nearly all the Axolotls
could have been compelled to undergo transformation.
I must not, however, omit to mention that
Herr Gehrig, the curator of our Zoological
Museum, bred a considerable number of larvæ
from the same brood as that with which Fräulein
v. Chauvin experimented, and that of these larvæ
six lived over the winter without undergoing metamorphosis.
They were always kept in deep water
and thus furnished the converse experiment to
those recorded above; they further prove that this
whole brood did not have a previous tendency to
undergo metamorphosis.
If these new facts are to be made use of to
explain the nature of this extraordinary process of
transformation in accordance with our present
conception, the data already known must in the
first place be called to our aid.
It has first to be established that Siredon
Mexicanus never, as far as we know, undergoes
metamorphosis in its native country. This Amphibian
is there only known in the Siredon form, a
statement which I have taken from De Saussure,225
who has himself observed the Axolotl in the Mexican
lakes. This naturalist never found a single
Amblystoma in the neighbourhood of the lakes,
“nevertheless the larva (Axolotl) is so common
there that it is brought into the market by thousands.”
De Saussure believes that in Mexico the
Axolotl does not undergo transformation.226 The
same statement is distinctly made by Cope,227 whose
specimens of Siredon Mexicanus bred in America,
even in captivity showed “no tendency to become
metamorphosed.” On the other hand Tegetmeier
observed228 that one out of five specimens obtained
from the Lake of Mexico underwent metamorphosis,
and this accordingly establishes the second fact,
viz. that the true Axolotl becomes transformed
under certain conditions into an Amblystoma when
in captivity.
This last remark would be superfluous if, as was
for a long time believed, the Paris Axolotls, of
which the metamorphosis was first observed and
which at the time made such a sensation, were
actually Siredon Mexicanus, i.e. the Siredon which
alone in its native country bears the name of
Axolotl. In his first communication Duméril was
himself of this opinion; he then termed the animal
“Siredon Mexicanus vel Humboldtii,”229 but subsequently,
in his amplified work230 on the transformation
of the Axolotl observed in the Jardin
des Plantes, he retracted this view, and after a
critical comparison of the five described species
of Siredon, he came to the conclusion that the
species in the possession of the Paris Museum was
probably Siredon Lichenoides (Baird). All the
transformations of Axolotls observed in Europe
must consequently be referred to this species,
since they were—at least as far as I know—all
derived from the Paris colony. My own experimental
specimens were also indirectly descended
from these.
Now it must be admitted that this does not
coincide with the fact that the Amblystoma form
which Duméril first obtained from his Axolotls
agreed with Cope’s species, A. Tigrinum, while on
the other hand we learn from Marsh231 that Siredon
Lichenoides (Baird), when it does undergo metamorphosis,
becomes transformed into Amblystoma
Mavortium (Baird).
Marsh found Siredon Lichenoides in mountain
lakes (7000 feet above the sea) in the southwest
of the United States (Wyoming Territory),
and obtained from them, by breeding in aquaria,
Amblystoma Mavortium (Baird). He considers
it indeed doubtful whether the Amphibian undergoes
this transformation in its native habitat,
although he certainly states this opinion without
rigorous proof on purely theoretical considerations,
because, according to his view, “the low temperature
is there less favourable.”232
If I throw doubt upon this last statement it is
simply because Amblystoma Mavortium is found
native in many parts of the United States, viz:—in
California, New Mexico, Texas, Kansas,
Nebraska, and Minnesota. It is indeed by no
means inconceivable that in the mountain lakes
where Marsh obtained this species, it may behave
differently with respect to metamorphosis than in
other habitats, and this appears probable from
certain observations upon Triton which will be
subsequently referred to.
Meanwhile, in the absence of further observations,
we must admit that the Paris Axolotls were
not Siredon Lichenoides, but some nearly allied and
probably new species. But little information is
furnished by observing the course of the transformation,
although it is at least established that this
Axolotl in its native habitat does not undergo
metamorphosis or does so as exceptionally as in
Europe. Unfortunately in his papers Duméril
gives no precise statement respecting the locality
of this species imported from “Mexico”—it is
probable that he was himself unacquainted with
it, so that I can only state on the authority of
Cope that Amblystoma has never been brought
from south of the provinces of Tamaulipas
and Chilhuahua, i.e. south of the Tropic of
Cancer.233
This last statement, however, gives no certainty
to the matter. Of much more importance is the
above-mentioned fact, that the true Axolotl of the
lakes about the city of Mexico does not, as a rule,
become transformed into an Amblystoma in that
locality, although this species in certain cases
undergoes metamorphosis when in confinement.
From this circumstance and from the fact that the
Paris Axolotl likewise experienced but a very small
percentage of metamorphosis in captivity, we may
venture to conclude that this species also, in its
native habitat, either does not become transformed
at all or does so only exceptionally.
But there is yet another body of facts which
come prominently into view on considering the
history of the transformations. I refer to the
existence of numerous species of Amblystoma in a
natural state. In the “Revision of the genera of
Salamandridæ,” published some years ago by
Strauch,234 this author, following Cope,235 gives
twenty species of Amblystoma as inhabiting North
America. Although some of these species are
based on single examples, and consequently, as
Strauch justly remarks, “may well have to be
reduced in the course of time,” there must nevertheless
always remain a large number of species
which live and propagate as true Amblystomas,
and of which the habitat extends from the latitude
of New York to that of New Mexico. There are
therefore true species of Siredon which regularly
assume the Amblystoma form under their natural
conditions of life, and which propagate in this
form, while, on the other hand, there are at least
two species which, under their existing natural conditions
of life, always propagate as Siredon. It is
but another mode of expression for the same facts
if we say that the Mexican Axolotl and the Paris
Siredon—whether this is Lichenoides or some other
species—stand at a lower grade of phyletic
development than those species of Amblystoma
which propagate in the salamander form. No one
can raise any objection to this statement, while the
alternative view maintained by all authors contains
a theory either expressed or implied which is, as I
believe, incorrect, viz. that the Mexican Axolotl
has remained at an inferior stage of phyletic
development.
All zoologists236 who have expressed an opinion
upon the transformation of the Axolotl, and
who are not, like the first observer of this fact,
embarrassed by Cuvier’s views as to the immutability
of species, regard the phenomenon as though
a species, which owing to some special conditions
had hitherto remained at a low stage of development,
had now through some other influences been
compelled to advance to a higher stage.
I believed for a long time that the phenomenon
could not otherwise be comprehended, so little was
I then in a position to bring all the facts into harmony
with this view. Thus in the year 1872 I
expressed myself as follows237:—“Why should not
a sudden change in all the conditions of life (transference
from Mexico to Paris) have a direct action
on the organization of the Axolotl, causing it suddenly
to reach a higher stage of development, such
as many of its allies have already attained, and
which obviously lies in the nature of its organization—a
stage which it would perhaps itself have
reached, although later, in its native country? Or
is it inconceivable that the sudden removal from
8000 feet above the sea (Mexican plateau) to the
altitude of Paris, may have given the respiratory
organs an impetus in the direction of the transformation
imminent? In all probability we have here
to do with the direct action of changed conditions
of life.”
That the substance of this last statement must
still hold good is obvious from the experiments previously
described, which show that by the application
of definite external influences, we have it to a
certain extent in our power to produce the transformation.
It is precisely in this last point that
there lies the new feature furnished by these experiments.
But are we also compelled to interpret the phenomenon
in the above manner? i.e. as a sudden
advance in the phyletic development of the species
occurring, so to speak, at one stroke? I believe
not.
What first made this view appear to me erroneous,
was the appearance of the living Amblystomas
bred from my Axolotl larvæ. These creatures
by no means differed from the Axolotls merely in
single characters, but they were distinct from
the latter in their entire aspect; they differed in
some measure in all their parts, in some but
slightly and in other parts strongly—in brief, they
had become quite different animals. In accordance
with this, their mode of life had become completely
modified; they no longer lived in the water,
but remained concealed by day among the damp
moss of the vivarium, coming forth only by night
in search of food in dry places.
I had been able to perceive the great difference
between the two stages of development from the
anatomical data with which I had long been
familiar, and which Duméril had made known with
respect to the structure of his Amblystomas. But
the collecting of numerous details gives no very
vivid picture of the metamorphosis; it was the
appearance of the living animal that first made me
conscious how deep-seated was the transformation
which we have here before us, and that this change
not merely affected those parts which would be
directly influenced by the change in the conditions
of life, such as the gills, but that most if not all
the parts of the animal underwent a transformation,
which could in part be well explained as morphological
adaptation to new conditions of life, and
partly as a consequence of this adaptation (correlative
changes), but could not possibly be regarded
as the sudden action of these changed
conditions.
Such at least is my view of the case, according
to which a per saltum development of the species
of such a kind as must here have taken place, is
quite inconceivable.
I may venture to assume that most observers
of the metamorphosis of Axolotl have, like myself,
not been hitherto aware of the extent of the
transformation, and it may thus be explained why
the theoretical bearings of the case have on all
sides been incorrectly conceived. We have here
obviously a quite extraordinary case of the first
order of importance. I believe that it can easily
be shown that the explanation of the history of the
metamorphosis of the Paris Axolotl which has
hitherto been pretty generally accepted, necessarily
comprises a very far-reaching principle. If this
interpretation is correct, then in my opinion must
also hold good the ideas of those who, like Kölliker,
Askenasy, Nägeli, and, among the philosophers,
Hartmann and Hübner, would refer the transformation
of species in the first instance to a power
innate in the organism, to an active, i.e. a self-urging
“law of development”—a phyletic vital
force.
Thus, if the Axolotls transformed into Amblystomas
are regarded as individuals which, impelled
by external influences, have anticipated the phyletic
development of the others, then this advance can
only be ascribed to a phyletic vital force, since the
transformation is sudden, and leaves no time for
gradual adaptation in the course of generations.
The indirect influence of the external conditions
of life, i.e. natural selection, is thus excluded from
the beginning. But the direct action of the
changed conditions of life by no means furnishes
an explanation of the complete transformation of
the whole structure, such as I have already alluded
to, and which I will now enter into more closely.
The differences between the Paris Axolotl and
its Amblystoma according to Duméril, Kölliker,
and my own observations are as follow:—
1. The gills disappear; the gill-clefts close up,
and of the branchial arches only the foremost remain,
the posterior ones disappearing. At the
same time the os hyoideum becomes changed
(Duméril).
2. The dorsal crest completely disappears
(Duméril).
3. The aquatic tail becomes transformed into
one like that of the salamanders (Duméril), which,
however, is not as in the salamander cylindrical,
but somewhat compressed laterally (Weismann).
4. The skin becomes yellowish white, irregularly
spotted on the sides and back (Duméril),
whilst at the same time its former grey-black
ground-colour changes into a shining greenish
black (Weismann); it loses, moreover, the slimy
secretion of the skin, and the cutaneous glands
become insignificant (Kölliker).
5. The eyes become prominent and the pupils
narrow (Kölliker), and eyelids capable of completely
covering the eyes are formed; in Axolotl
only a narrow annular fold surrounds the eyes, so
that these cannot be closed (Weismann).
6. The toes become narrowed and lose their
skin-like appendages (Kölliker), or more precisely,
the half webs which connect the proximal extremities
of the toes on all the feet (Weismann).
7. The teeth are disposed in this species, as in
all other Amblystomæ, in transverse series; whilst
in Axolotl, as in Triton larvæ, they are arranged
at the sides of the jaw in the form of a curved
arch-like band beset with several rows of teeth.238
(Duméril. See his fig., loc. cit. p. 279).
8. In Axolotl the lower jaw, in addition to the
teeth on the upper edge of the bone, also bears
“de très petites dents disposées sur plusieurs
rangs;” these last disappear after metamorphosis
(Duméril). I will add that the persistent teeth
belong to the os dentale of the lower jaw, and
those that are shed to the os operculare.239
9. The surface of the posterior moveable part
of the body is slightly concave both before and
after transformation; the anterior part is, however,
less concave in Amblystoma than in Siredon
(Duméril).
I have not yet been able to verify Duméril’s 7th
and 9th statements, as I did not want to kill any
of my living Amblystomas,240 simply in order to
confirm the observations of a naturalist in whom
one may certainly place complete confidence.
Neither have I as yet observed the transformation
of the branchial arches, but all the other statements
of Kölliker and Duméril I can entirely corroborate.
The structural differences between Axolotl and
Amblystoma are considerably greater and of more
importance than those between allied genera, or
indeed than between the families of the Urodela.
The genus Siredon undoubtedly belongs to a
different sub-order to the genus Amblystoma into
which it occasionally becomes transformed.
Strauch, the most recent systematic worker at
this group, distinguishes the sub-order Salamandrida
from that of the Ichthyodea by the possession
of eyelids, and by the situation of the palatine
teeth in single rows on the posterior edge of
the palatal bone: in Ichthyodea the eyelids are
wanting and the palatine teeth are either “situated
on the anterior edge of the palatal bone,” or
“cover the whole surface of the palatal plates in
brush-like tufts.”
How is it possible to regard such widely divergent
anatomical characters as changes suddenly
produced by the action (but once exerted) of deviating
conditions of life? Hand in hand with the
shedding of the old and the appearance of new
palatine teeth, there occurs a change in the anatomical
structure of the vertebral column, and also—as
we may fairly conclude from Kölliker’s
correct observation of the cessation of the slimy
secretion—in the histological structure of the skin.
Who would undertake to explain all these profound
modifications as the direct and sudden
action of certain external influences? And if any
one were inclined to explain such changes as a
consequence of the disappearance of the gills, i.e.
as correlative changes, what else is such a correlation
than the phyletic vital force under another
name?
If from one change arising from the direct
action of external agencies, the whole body can in
two days become transformed in all its parts, in
the precise manner which appears best adapted for
the new conditions of life under which it is henceforward
to exist, then the word “correlation” is
only a phrase which explains nothing, but which
prevents any attempt at a better explanation, and
it would be preferable to profess simply the belief
in a phyletic vital force.
Moreover, it is hardly permissible to seek such
an explanation, since Urodela are known which
have no gills in the adult state, and which nevertheless
possess all the other characters of the Ichthyodea,
viz. want of eyelids, characteristic palatine
teeth, and the tongue bone. This is the case
with the genera Amphiuma (Linn.), Menopoma
(Harl.), and Cryptobranchus (v. d. Hoev.). The two
first genera, as is known, still possess gill-clefts,
but Cryptobranchus has even lost these clefts,
which, as in Amblystoma, are overgrown by skin;
nevertheless Cryptobranchus is, according to the
concurrent testimony of all systematists, a true
salamander in habits, tongue bone, palatine
teeth,241 &c. It must further be added that the
Axolotl itself can lose the gills without thereby
becoming transformed into an Amblystoma. I
have previously mentioned that in Axolotls which
were kept in shallow water the gills frequently
became diminutive, and it also sometimes happens
that they completely shrivel up. I possess an
Axolotl preserved in alcohol in which the gills have
shrivelled up into small irregular bunches, and the
dorsal crest is also so completely absent that its
place is occupied by a long furrow, and even on the
tail the crest has entirely disappeared from the
lower edge and about half from the upper edge.
Notwithstanding this, the creature is widely removed
from Amblystoma in structure; it possesses
the arched branchial apparatus, the palatine teeth,
the skin, &c., of the Axolotl.
These facts prove, therefore, that the shedding
of the gills by no means always entails all the
other modifications which we observe in the metamorphosis
of Axolotl, so that these modifications
are thus not by any means the necessary and
immediate consequence of such gill shedding.
Whether these modifications will occur after a
long series of generations—whether the successors
of Cryptobranchus will also one day acquire the
salamandriform structure is another question, and
one which I could not exactly answer in the
negative. But this question does not here come
into consideration, as we are now only concerned
with the immediate result of the shedding of the
gills.
The problem appears therefore to be as follows:—Either
the hitherto received interpretation
of the transformational history of the Axolotl as a
further development of the species is incorrect, or
else the case of Axolotl incontestably proves the
existence of a phyletic vital force.
We have now to ask whether the facts of this
transformational history are not capable of another
explanation.
I believe that this is certainly possible, and that
another interpretation can be shown to be correct
with some degree of probability.
I am of opinion that those Amblystomas which
have been developed in captivity in certain instances
from Siredon Mexicanus (S. Pisciformis),
as well as from the Paris Axolotls, are not progressive,
but reversion forms; I believe that the
Axolotls which now inhabit the Mexican lakes
were Amblystomas at a former geological (or
better, zoological) epoch, but that owing to
changes in their conditions of life, they have
reverted to the earlier perennibranchiate stage.
I was undoubtedly first led to this conception
by the results which arose from my studies on the
seasonal dimorphism of butterflies.242 In this case
we were also concerned with the two different
forms under which one and the same species
appears, and of which it was shown to be probable
that the one is phyletically older than the other.
The younger summer form, according to my view,
has arisen, through the gradual amelioration of the
climate, from the winter form, which at an earlier
zoological epoch was the only one in existence;
but the latter, the primary form, has not for this
reason ceased to exist, but now alternates in each
year as a winter form with the secondary summer
form.
Now with seasonally dimorphic butterflies, it
was easily possible to induce the summer brood to
assume the winter form by exposing their pupæ
for a long time to a low temperature; and it was
shown to be highly probable that this abrupt and
often very extensive change or transformation,
only apparently takes place suddenly, and is but
the apparent result of the action of cold upon this
generation, whilst in fact it depends upon reversion
to the primary form of the species, so that the
low temperature, which is only once applied, gives
but the impetus to reversion, and is not the true
cause of the transformation. This cause must
rather be sought in the long continued action of
the cold to which the ancestors of our existing
butterflies were subjected for thousands of generations,
and of which the final result is the winter
form.
If we assume for an instant that my interpretation
of the transformation of Axolotl as just offered
is correct, we should have conditions in many
respects analagous to those of seasonal dimorphism.
It is true that in this case the two forms
no longer alternate regularly with each other, but
the primary form may occasionally appear instead
of the secondary form, owing to the action of
external conditions.
Just as in the case of seasonal dimorphism it
is possible to compel the summer generation to
abandon the summer form, and to assume the
winter guise by the action of cold; so in the
present case we are able to induce the Axolotl to
adopt the Amblystoma form by making aërial
respiration compulsory at a certain stage of life;
and further, just as in seasonal dimorphism it can
be shown that this artificially produced change is
only apparently an abrupt transformation, and is
actually a reversion to the much older winter
form; so here we have not an actual, but only an
apparent remodelling of the species—a reversion
to the phyletically older form.
This certainly appears a paradox, inasmuch as
a form here arises by reversion which must yet
undoubtedly rank as the more highly developed.
I believe, however, that much which seems paradoxical
in this statement will disappear on further
examination.
It must in the first place be taken into consideration
that the phyletic development of species
need not by any means always take place by
advancement. We have indeed many cases of
retrogressive development, although in a somewhat
different sense, as with parasites and those
forms which have degenerated from free locomotion
to a sedentary mode of life.243 I do not
confuse this kind of retrogressive development,
arising from the arrest of certain organs and
systems of organs, with true reversion. The
latter is a return to a form which has already been
once in existence; but in the former case, in spite
of all simplification of the organization, some
entirely new feature always comes into existence.
But I am not able to see any absurdity in the
assumption that even true reversion, whether of a
whole species or of the individuals of a certain
district, may be regarded as possible, and I require
no further concession. Why, for example, should
it be inconceivable that at a very remote period
the Axolotl was adapted to a life on land; that
through the direct and indirect action of changed
conditions of life it gradually acquired the salamander
form, but that subsequently, through new
and unfavourable changes in the conditions of
life, it again relapsed to the older form, or at least
to one nearly related thereto?
At any rate such an assumption contains
nothing opposed to known facts, but can be supported
in many ways, and finally it commends
itself, at least in my opinion, as offering the only
admissible explanation of the facts before us.
The existence of a whole series of species of
Amblystoma, as already mentioned, at once shows
that species of Siredon can become elevated into
the salamander form, and can propagate regularly
in this state, and further, that this phyletic advance
has already actually taken place in many
species.
That degeneration may also occur from this
high stage to a lower stage of development, is
shown by many observations on our water-salamanders.
It is known that under certain circumstances
Tritons, as it is generally expressed, become
“sexually mature in the larval condition.”
In the year 1864 De Filippi244 found fifty Tritons
in a pool at Andermatten, in the neighbourhood of
Puneigen, and of these only two showed the
structure of the adult water-salamander; all the
others still possessed gills, but notwithstanding this,
they agreed in both sexes, in size and in the development
of the sexual organs, with mature animals.
De Filippi established that these “sexually mature
larvæ” not only resembled larvæ externally through
the possession of gills, but that they also possessed
all the other anatomical characters of the
larvæ, i.e. the characteristic bunches of palatine
teeth situated on both sides in the position of the
subsequent single rows, and a vertebral column
represented throughout its whole length by the
chorda dorsalis.
According to my view this would be a case of
the reversion of the Triton to the immediately anterior
phyletic stage, i.e. to the perennibranchiate
stage, and in the present instance the majority of
zoologists who take their stand by the theory of
descent, would certainly concur in this view. I
should at least consider it to be a useless play
upon words did we here speak of larval reproduction,
and thereby believe that we had explained
something. The animal certainly becomes sexually
mature in the same condition as that in which it first
appears as a larva, but we first get an insight into
the nature of this process by considering that this
so-called “sexually mature larva” has the precise
structure which must have been possessed by the
preceding phyletic stage of the species, and that
an individual reversion to the older phyletic stage
of the species is consequently before us. I maintain
that Duméril is in error in regarding this case
of the Triton as parallel with the true larval reproduction
of Wagner’s Cecidomyia larva. In
this last case it is certainly not reversion to an
older phyletic stage that confers the power of
reproduction upon the larvæ, since the latter do
not represent an older phyletic stage of the
species, but must have arisen contemporaneously
with this last stage. The enormous structural
difference between the larvæ and the imagines is
not explained by the latter having arisen from the
former supplementarily as a finished production,
but by both having been contemporaneously
adapted to continually diverging conditions of
life.245 Considered phyletically, these larvæ are by
no means necessarily transitional to the origination
of the flies. They could have been quite different
without the form of the imagines having been
thereby modified, since the stages of insect metamorphosis
vary independently of each other in
accordance with the conditions of life to which
they are subjected, and exert scarcely any, or only
a very small form-determining influence upon each
other, as has been amply proved in the preceding
essay. In any case the power of these larvæ (the
Cecidomyiæ) to propagate themselves asexually
was first acquired as a secondary character, as
appears from the fact that there exist numerous
species of the same genus which do not “nurse.”
In the form which they now possess they could
never have played the part of the final stage of
the ontogeny, nor could they formerly have possessed
the power of sexual reproduction.246 In
brief, we are here concerned with true larval reproduction,
whilst in Triton we have reversion to
an older phyletic stage.247
I cannot agree with my friend Professor Haeckel
when he occasionally designates the reversion of
the Tritons as an “adaptation” to a purely aqueous
existence.248 We could here only speak of “adaptation”
if we took the word in a quite different sense
to that in which it was first introduced into
science by Darwin and Wallace. These naturalists
thereby designate a gradual bodily transformation
appearing in the course of generations in correspondence
with the new requirements of altered
conditions of life or, in other words, the action of
natural selection, and not the result of a suddenly
and direct acting transforming cause exerted but
once on a generation.
Just because the word “adaptation” can be
used in ordinary language in many senses, it is
desirable that it should have only one precise signification,
and above all that we should not speak
of adaptation where scarcely any morphological
change occurs, but only a kind of functional
change in the sense used by Dohrn.249 This is the
case for example, when Forel250 shows that fresh
water Pulmonifera, the organization of which is
attributed to the direct respiration of air, can
nevertheless become settled in the greatest depths
of mountain lakes through their lungs being again
employed as gills. That not the least change in
the lungs hereby takes place is shown by the
observations of Von Siebold,251 who saw the shallow
water Pulmonifera using their lungs alternately
for direct aërial and aquatic respiration, according
to the amount of air contained in the water. If
with Von Siebold we merely apply the word
“adaptation” to such cases, this expression would
lose the special sense which it originally conveyed,
and the word would have to be abandoned as a
terminus technicus; still, such cases may perhaps
be spoken of as physiological adaptation.
In any case the reproductive “larvæ” of the
Tritons as little present a case of true adaptation
as the Axolotl, which occasionally becomes transformed
into an Amblystoma. In both cases the
transformation referred to is by no means indispensable
to the life of the individual. Mature Tritons
(devoid of gills) can exist, as I have myself seen, for
many months, and probably also for a year in deep
water, although adapted for purely pulmonary
respiration; whilst Axolotls, as I have already mentioned,
can live well for a year in shallow water
poor in air. If their gills by this means become
shrivelled up or completely disappear, even this is
not adaptation in the Darwinian sense, but the
effect of directly acting external influences, and
chiefly of diminished use.
A case entirely analagous to that of Filippi’s
was observed by Jullien in 1869. Four female larvæ
of Lissotriton Punctatus (Bell)—(synonymous
with Triton Tæniatus, Schnd.), taken from a pool,
proved to be sexually mature. They contained
mature eggs in their ovaria ready for laying, and
two of them actually deposited eggs. Four male
larvæ found in the same pool, appeared to be
equally developed with respect to size, but their
testicles contained no free spermatozoa, but only
sperm-cells.252
I have met with a third case of a similar kind
mentioned by Leydig in his memoir, rich in interesting
details, “on the tailed Amphibians of
the Wurtemburg fauna.”253 Schreibers, the former
director of the Vienna Museum, also found
“larvæ” of Tritons with well-developed gills, but
of the size of the “adult male individuals,” and,
as shown by anatomical investigation, with well
“developed sexual organs,” the ovaria especially
being distended with eggs.
It is thus established that species which long
ago reached the salamander stage in phyletic
development, may occasionally degenerate to the
perennibranchiate stage. This fact obviously
makes my conception of the Axolotl as a reversion
form appear much less paradoxical—indeed, the
cases of reversion in Triton are precisely analagous
to the process which I suppose to have taken
place in the Axolotl. We have only to substitute
Amblystomas for Tritons, to imagine the pool in
which De Filippi found his “sexually mature Triton
larvæ” enlarged to the size of the Lake of Mexico,
and to conceive the unknown, and perhaps here
transitory, causes of the reversion to be permanent,
and we have all that is necessary, so far as
we at present know, for the restoration of the
Axolotl; we obtain a perennibranchiate population
of the lake.
It has not yet been determined whether the
perennibranchiate form of the Triton actually prevailed
permanently in De Filippi’s pool, since, so
far as I know, this has not since been examined.
Let us, however, assume for an instant that this
is really the case, and that there exists at that
spot a colony of sexually reproductive perennibranchiate
Tritons: should we wonder if a true
Triton occasionally appeared among their progeny,
or if we were able to induce the majority of the
individuals of this brood to become metamorphosed
into Tritons by keeping them in shallow water?
According to my view this is precisely the case of
the Mexican Axolotl.
I need not, however, restrict myself to this in
order to support my hypothesis, but must also
directly combat the view hitherto received, since
the latter is in contradiction with facts.
Did there really exist in the Axolotl a tendency
to sudden phyletic advancement, then one fact
would remain quite incomprehensible, viz. the
sterility of the Amblystomas.
Out of about thirty Amblystomas obtained by
Duméril down to the year 1870, there was not one
in a state of sexual maturity; neither copulation
nor deposition of eggs took place, and the anatomical
investigation of single specimens showed that
the eggs were immature, and that the spermatozoa,
although present, were without the undulating membrane
characteristic of the salamanders, but were
not devoid of all power of movement, only, as established
by Quatrefages, were “incompletely motile.”254
So also the five Amblystomas about which I
have been writing, show up to the present time no
appearance of reproduction.
The objection raised by Sacc,255 that the sterility
of the Amblystomas bred from Axolotls is attributable
to “bad nourishment,” is obviously of
but little avail. How is it that the Axolotls, which
are fed in a precisely similar manner, propagate so
readily? Moreover, I am able to expressly assert
that my Amblystomas were very well fed. It is
true that they have as yet scarcely reached the
age of two years, but the Axolotl propagates
freely in the second year, and some of Duméril’s
Amblystomas were five years old in 1870.
This fact of the sterility is strongly opposed to
the idea that these Amblystomas are the regular
precursors of the phyletically advancing genus
Siredon.256 I will by no means assert that my
theory of reversion actually explains the sterility,
but it is at least not directly opposed to it. Mere
reversion forms may die off without propagating
themselves; but a new form called forth by the
action of a phyletic vital force should not be sterile,
because this is the precise “aim” which the vital
force had in view. The conception of a vital force
comprises that of teleology.
The sterility of Amblystoma moreover, although
not completely explicable from our standpoint, can
be shown to be a phenomenon not entirely isolated.
In the above mentioned case of Lissotriton Punctatus,
the female “larvæ” were certainly sexually
mature and laid eggs, but the males of the same
period contained in their testicles no fully developed
spermatozoa.
Other cases of this kind are unknown to me;
at the time when I made the experiments with
butterflies already recorded (see the first essay),
this point of view was remote, and I therefore
neglected to examine the artificially bred reversion
forms with respect to their organs of reproduction.
But general considerations lead to the supposition
that atavistic forms may easily remain sterile.
Darwin257 finds the proximate causes of sterility
in the first place in the action of widely diverging
conditions of life, and in the next place in the
crossing of individuals widely different in constitution.
Now it is certainly deviating conditions of
life which lead to the metamorphosis of the Axolotl,
and from this point of view it cannot be surprising
if we find those individuals sterile which show
themselves so especially affected by these changed
conditions as to revert to the salamander form.
By this it is not in any way meant to be asserted
that reversion is invariably accompanied by sterility,
and one cannot raise as an objection to my interpretation
of the metamorphosis of the Axolotl, that
a reproductive colony of Axolotls could never have
arisen by reversion. On the contrary, Jullien’s
egg-depositing female Triton larvæ show that also
with reversion the power of reproduction may be
completely preserved.258 From the above-mentioned
general causes of sterility, it may even be inferred
that fertility can be lost in different degrees, and it
can be further understood to a certain extent why
this fertility is more completely lost by reversion
to the Amblystoma, than by the reversion of the
Triton to the perennibranchiate form.
If in these cases the reversion is brought about
by a change in the conditions of life, we may
perhaps suppose that the magnitude of this change
would determine the degree of fertility, and the
preservation of the reversion form. Still more,
however, would the fertility be influenced by the
extent of the morphological difference resulting
from the reversion. We know that the blending
of very different constitutions (e.g. the crossing of
different species) produces sterility. Something
similar results from the sudden reversion to a stage
of development widely different in its whole
structure. Here also we have in a certain sense
the union of two very different constitutions in one
individual—a kind of crossing.
From this point of view it can in some measure
be comprehended why sterility may be a result of
reversion; on the other hand, we thereby obtain no
explanation why, with the same amount of morphological
difference, in one case complete sterility, and
in another relative fertility occurs. The morphological
difference between Axolotl and Amblystoma
is exactly the same as between Triton and its
“sexually mature larva;” the difference between
the two cases of reversion depends entirely upon
the direction of the leap, that taken in the former
case being precisely opposite in direction to that
taken in the latter.
Herein might be sought the explanation of the
different strength with which the reproductive
power is affected; not indeed in the direction of
the leap itself, but in the differences in the ontogeny
which are determined by the differences in the
direction of the leap. The reversion of the
Triton to an older phyletic stage coincides with
the arrest at a younger ontogenetic stage; or, in
other words, the older stage of the phylogeny to
which reversion takes place is still entirely
comprised in the ontogeny of each individual.
Each Triton is perennibranchiate throughout a
long period of its life; the reverting individual
simply reverts to the older phyletic stage by
remaining at the larval stage of its individual
development.
But it is quite different with the reversion of the
Axolotl to the formerly acquired, but long since
abandoned Amblystoma form. This is not retained
in the ontogeny of Axolotl, but has been completely
lost; for a long series of generations—so
must we suppose—the ontogeny has always only
attained to the perennibranchiate form. Now if at
the present time certain individuals were compelled
to revert to the Amblystoma form, certainly no
greater leap would have been made from a morphological
point of view, than in the reversion of
Triton to the perennibranchiate form, but at the
same time the leap would be in another direction,
viz. over a long series of generations back to a form
which the species had not produced for a long period,
and which had to a certain extent become foreign
to it. We should thus have here also the grafting
of a widely different constitution upon that of the
Axolotl, or, if one prefers it, the commingling of
two widely different constitutions.
Of course I am far from wishing to pretend that
this “explanation” is exact; it is nothing more
than an attempt to point out the direction in which
the causes affecting the reproductive powers in
different degrees are to be looked for. A deeper
penetration into and special demonstration of the
manner in which these causes bring about such
results, must be reserved for a future period. For
the present it must suffice to have indicated that
there is an essential distinction between the two
kinds of reversion, and to have made it to some
extent comprehensible that this distinction may be
the determining impulse with respect to the question
of sterility. Perhaps the law here concealed from
us may one day be thus formulated:—Atavistic
individuals lose the power of reproduction the more
completely, the greater the number of generations
of their ancestors whose ontogeny no longer comprises
the phyletically older stage to which the
reversion takes place.
The hypothesis which interprets the transformation
of the Axolotl as a case of reversion, thus holds
out the possibility of our being able to comprehend
the sterility of the Amblystomas arising in this
manner, whilst, on the other hand, for the adherents
of a phyletic vital force, not only is this observed
sterility as Duméril expresses it “un véritable
énigme scientifique,” but an absolute paradox.
We should expect such a directive and inciting
principle to call into existence new forms having
vitality and not destined to perish, the more so
when it is concerned with a combination of
structural characters which, when originating in
another manner (viz. from other species of Siredon),
have long since shown themselves to have vitality
and reproductive power. We are indeed acquainted
with species of Amblystoma which propagate as
such, and each of which arises from an Axolotl-like
larva. Thus we cannot regard the sterile Amblystomas
produced by the Paris Axolotls as abortive
attempts of a vital force—an interpretation which
is certainly in itself already sufficiently rash.
Now if it be asked what change in the conditions
of life could have led to the reversion in the Lake
of Mexico259 of the Amblystoma to the Siredon
form, I must admit that I can only offer a conjectural
reply, having but a conditional value so
long as it is not supported by a precise knowledge
of the conditions there obtaining, and of the habits
both of the Axolotl and of the Amblystoma.
It may be supposed generally that reversion is
brought about by the same external conditions as
those which formerly produced the perennibranchiate
stage. This supposition is in the first place
supported by the experiments here recorded, since
it is evidently the inducement to aërial respiration
which causes the young Axolotl to revert to the
Amblystoma form, i.e. the inciting cause under
whose domineering influence the Amblystoma form
must have arisen.
Here again the case is quite similar to that of
seasonally dimorphic butterflies. Reversion of
the summer brood to the winter form is there most
easily caused by the action of cold, i.e. by the
same influence as that under whose sway the winter
form was developed.
We know indeed that reversion may also arise
by the crossing of races and species, and I have
attempted to show that reversion in butterflies may
also be brought about by other influences than
cold; but still the most probable supposition
obviously is, that reversion would be caused by the
persistent action of the same influences as those
which in a certain sense created the perennibranchiate
form. That the latter was produced under
the influence of an aquatic life there can be no
doubt, and thus, in accordance with my supposition,
the hypothetical Amblystoma Mexicanum, the
supposed ancestral form of the Axolotl of the
Mexican Lake, might have been caused to revert to
the perennibranchiate form by a reduction in the
possibilities of its living upon land, and by its being
compelled to frequent the water.
I will not here return to the consideration of
every other opinion ab initio. It is very advisable
to distinguish between the mere impulses which
are able to produce sudden reversion, and between
actual transforming causes which result directly or
indirectly in the remodelling of a species. Thus,
it is conceivable à priori that reversion may occur
by the action of an inciting cause having nothing
to do with the origin of the phyletically older form.
Temperature can certainly have played no part, or
only a very small part, in the formation of the
perennibranchiate form; nevertheless cold may
well have been one of the inciting causes which
induced the Amblystoma at one time to revert
to the Siredon form, and we cannot at present
consider De Saussure to be incorrect when he
maintains that the low temperature of the Mexican
winter might prevent that transformation (of the
Axolotl into the Amblystoma) which would occur
“in the warm reptile-house” of the Jardin des
Plantes. He supports this view by stating that
“Tschudi has found the Amblystoma” (of course
another species) “in the hottest parts of the United
States.” “On the Mexican plateau, however, it
snows every winter, and if the lake does not actually
freeze, its temperature must fall very considerably
in the shallowest parts.”
But although this view is not opposed by any
theoretical considerations, I still hold it to be incorrect.
I doubt whether it is temperature that
has brought about the reverse transformation of
the Amblystoma into the Axolotl, or which, according
to De Saussure’s conception, at the present
time prevents the transformation of the Axolotl in
the Lake of Mexico. I doubt this because Amblystomas
are now known from all parts of the United
States as far north as New York, a proof that a
winter cold considerably greater than that of the
Mexican plateau is no hindrance to the metamorphosis
of the Axolotl, and that the genus does not
show itself to be in this respect more sensitive than
our native genera of Salamandridæ.
The following observations of De Saussure, in
which he calls attention to the nature of the
Mexican Lake, appear to me to be more worthy of
consideration:—“The bottom of this lake is
shallow, and one passes imperceptibly from the
lake into extensive marshy regions before reaching
solid ground; perhaps this circumstance makes
the Axolotl incapable of reaching dry land, and
prevents the transformation.”
In any case the Lake of Mexico offers very
peculiar conditions for Amphibian life. My
esteemed friend Dr. v. Frantzius has called my
attention to the fact that this lake—as well as many
other Mexican lakes—is slightly saline. At the
time of the conquest of Mexico by Ferdinand
Cortez, this circumstance led to the final surrender
of the city, as the Spaniards cut off the supply of
water to the besieged, and the water of the lake is
undrinkable. The ancient Mexicans had laid down
water-conduits from the distant mountains, and the
city is still supplied with water brought through
conduits.
Now this saltness cannot in itself be the cause
of the degeneration to the perennibranchiate form,
but it may well be so in combination with other
pecularities of the lake. The narrowest part of the
lake is the eastern, and it is only in this part that
the Axolotl lives. Now in winter, violent easterly
gales rush down from the mountains and blow
continuously, driving the water before them to such
an extent that it becomes heaped up in the western
portion of the lake, where it frequently causes
floods, whilst 2000 feet of the shallow eastern
shore are often laid completely dry.260
Now if we consider these two peculiarities, viz.
salineness and periodical drying up of a part of the
bottom of the lake through continuous gales, we
certainly have for the Axolotl, conditions of life
which are only to be found in few species. One
might certainly attempt to apply these facts in a
quite opposite sense, and to regard them as
unfavourable to my theory, since the retreat of the
water from a great portion of the bottom of the
lake would—so one might think—rather facilitate
transition to a life upon land, and indeed compel
the adoption of such a mode of existence. But
we should thus forget that the exposed bottom of
the lake is a sterile surface without food or place
of concealment, and, above all, without vegetation;
and further, that owing to the considerable salineness
of the water (specific gravity = 1.0215),261 the
whole of the exposed surface must be incrusted
with salt, a circumstance which would render it
quite impossible for the creatures to feed upon
land. Sodic chloride and carbonate are dissolved
in the water in such considerable quantities, that
they are regularly deposited upon the shores of
the lake as a crust, which is collected during the
dry season of the year and sent into the market
under the name of “tequisquite” (Mühlenpfordt).262
Thus the supposition is not wanting in support,
that peculiar conditions make it more difficult for
the creature to obtain its food upon land than in
the water, and this alone may have been sufficient
to have induced it to acquire the habits of a purely
aquatic existence, and thus to revert to the perennibranchiate
or Ichthyodeous form.
But enough of supposition. We must not complain
that we are unable from afar to discover with
precision the causes which compelled the Axolotl
to abandon the Amblystoma stage, as long as we
are not able to explain the much nearer cases of
reversion in Filippi’s and Jullien’s Tritons; nevertheless,
in these cases also, the causes affecting
the whole colony of Tritons must be general,
since—at least in the case noticed by Filippi—the
greater majority of the individuals remained in the
larval condition. Experiments with Triton larvæ
could throw greater light upon this subject; it
would have in the first place to be established
whether reversion could be artificially induced, and
if so, by what influences.
From the previously mentioned experiments with
butterflies, as well as from the results obtained with
Axolotls, we should expect that in Tritons, reversion
to the Ichthyodeous form would take place if we
allowed the inciting cause, viz. the bathing of the
gills and of the whole body with water, to act persistently,
and at the same time withheld that influence
under whose action the salamander form
became developed, viz. the bathing of the gills,
the skin, and the surfaces of the lungs with air.
Old experiments of this kind are to be met with,
but they were never carried on for a sufficient time
to entirely allay the suspicion, that the specimens
concerned would perhaps have undergone the
ordinary metamorphosis if their existence had been
prolonged.
Thus, Schreibers263 relates that “by confining
tadpoles of the salamander found at large in their
last stage of growth, under water by means of an
arrangement (net?), and feeding them with finely
chopped earthworms, he was able to keep them for
several months—and indeed throughout the winter—in
this condition, and in this way to forcibly defer
their final change, and their transition from the tadpole
stage to that of the perfected creature during
this period.” It is not stated whether the animals
finally underwent transformation, so that it cannot be
decided whether we have here a case of reversion
or simply one of retarded development. That
metamorphosis may occur after a long period of
time, is shown by experiments upon the tadpole
of Pelobates conducted by Professor Langer in
Vienna.264 The creatures were kept in deep water
in such a manner that they were not able to land,
and by this means three out of a large number
of individuals had their metamorphosis delayed
till the second summer; notwithstanding this,
transformation then occurred.
It cannot be objected to my reversion hypothesis,
that it opposes on the one side what on the other
it postulates, viz. a per saltum change of structure.
Reversion is characterized by the sudden acquisition
of an older, i.e. a formerly existing phyletic
stage. That reversion occurs is a fact, whilst
nobody has hitherto been able to prove, or even to
make probable, that a stage of the future (sit
venia verbo) has been attained at once (per
saltum).
Now if it is possible to find influences in the
present conditions of life of the Axolotl which make
it difficult or quite impossible for it to live upon
land, and which therefore appear as incentives to
the reversion to the Ichthyodeous form, the other
portion of my hypothesis—the assumption that the
Axolotl had become an Amblystoma at a former
period—can also be supported by facts.
We know from Humboldt265 that the level of
the Lake of Mexico at a comparatively recent
period was considerably higher than at present.
We know further that the Mexican plateau was
covered with forest, which has now been destroyed
wherever there are human, and especially Spanish
settlements. Now if we suppose that at some
post-glacial period the mountain forests extended
to the borders of the lake, at that time deep, with
precipitous sides and much less saline, not only
should we thus have presented different conditions
of life to those at present existing, but also such
as would be most favourable for the development
of a species of salamander.
On the whole, I believe that my attempt to
explain the exceptional metamorphosis of the
Axolotl of the Mexican lake cannot be objected to
as being a too airy phantasy. In any case it is the
only possible explanation which can be opposed to
that which supposes that the occasional transformation
of the Axolotl is not reversion, but an
attempt at advancement. This last assumption
must, in my judgment, be rejected on purely
theoretical grounds by those who hold that a
sudden transformation of a species, when connected
with adaptation to new conditions of life, is
inconceivable—by those who regard adaptation,
not as the sudden work of a magic power, but as
the end result of a long series of natural, although
minute and imperceptible causes.
If my interpretation of the facts be correct, there
arises certain consequences which I may here
briefly mention in conclusion.
First, with regard to more obvious results. If
Siredon Mexicanus, Shaw, only by occasional reversion
assumes the Amblystoma form, and never,
or only exceptionally, propagates as such, but only
as Siredon, the more recent systematists are not
justified in striking out the genus Siredon and in
placing S. Mexicanus as an undeveloped form in
the genus Amblystoma. So long as there exists
not one only, but several species of Siredon which
as such regularly propagate themselves, the genus
exists; and although we would not deprive systematists
of all hope of these species of Siredon
being one day re-elevated to Amblystomæ, it nevertheless
better accords with the actually existing
state of affairs if we allow the genus Siredon to
remain as before among the genera of Salamandrina,
and to include therein all those species
which, like the Paris Axolotl, S. Mexicanus, Shaw,
and probably also S. Lichenoides, Baird, only exceptionally,
or through artificial influences, assume
the Amblystoma form, but without propagating
regularly in this condition. On the other hand, we
should correctly comprise under the genus Amblystoma
all those species which propagate in this
state regularly, and in which the perennibranchiate
stage occurs only as a larval condition.
To arrive at a decision in single cases would
chiefly concern the American naturalists, whose
ever increasing activity may lead us to hope soon
for a closer investigation of the reproduction of
the numerous species of Amblystoma of their
native country. I should rejoice if the facts and
arguments which I have here offered should give
an impetus to such researches.
The second consequence to which I may refer,
is of a purely theoretical nature, and concerns a
corollary to the “fundamental biogenetic law”
first enunciated by Fritz Müller and Haeckel.
This, as is well known, consists of the following
law:—The ontogeny comprises the phylogeny,
more or less compressed and more or less modified.
Now according to this law, each step in phyletic
development when replaced by a later one, must
remain preserved in the ontogeny, and must therefore
appear at the present time as an ontogenetic
stage in the development of each individual. But
my interpretation of the transformation of the
Axolotl appears to stand in contradiction to this,
since the Axolotl, which at a former period was an
Amblystoma, retains nothing of the latter in its
ontogeny. The contradiction is, however, only
apparent. As long as we are concerned with an
actual advance in development, and therefore with
the attainment of a new step never formerly
reached, the older stages will be found in the
ontogeny. But this is not the case when the new
stage is not an actual novelty, but formerly represented
the final stage of the individual development;
or, in other words, when we are concerned
with the reversion, not of single individuals, but of
the species as such, to the preceding phyletic
stage, i.e. with a phyletic degeneration of the
species. In this case the former end-stage of the
ontogeny would be simply eliminated, and we
should then only be able to recognize its former
existence by its occasional appearance in a reversion
form. Thus, under certain conditions the
Triton sinks back to the perennibranchiate stage;
not in such a manner that the individual first
becomes a Triton and then undergoes perennibranchiate
re-modification, but simply, as I have
already shown above, by its remaining at the
Ichthyodeous stage and no longer attaining to the
Salamander form. So also, according to my
hypothesis, the salamandrine Amblystoma Mexicanum,
formerly inhabiting the shores of the
Lake of Mexico, has degenerated to the perennibranchiate
stage, and the only trace that remains
to us of its former developmental status is the
tendency, more or less retained in each individual,
to again ascend to the salamander stage under
favourable conditions.
The third and last consequence which my interpretation
of the facts entails, is the change in
the part played by reversion in organic nature.
Whilst atavistic forms have hitherto been known
only as isolated and exceptional cases, interesting
indeed in the highest degree, but devoid of significance
in the course of the development of organic
nature, a real importance in this last respect
must now be attached to them.
I may assume that reversion can in two ways
be effectual for the preservation or re-establishment
of a living form. In the first place, where,
as in Axolotl, the new and organically higher
form becomes untenable through external influences,
instead of simply perishing—since advancement
in another direction does not appear to
be possible—a reversion of the species to the
older and more lowly organized stage occurs. In
the second place, the older phyletic form may
not be abandoned while a newer form is being
developed therefrom, but the former may alternate
with the latter, as we see in the case of seasonally
dimorphic butterflies. It can hardly be objected
if I regard the alternation of the summer and
winter form in this case as a periodic reversion to
the phyletically older (winter) form.
Although the reversion of an entire species,
such as I suppose to have been the case with the
Axolotl, may be of rare occurrence, this is certainly
not the case with periodic or cyclical reversion;
the latter plays a very important part in
the development of the various forms of alternating
or cyclical propagation.266
In the previous portion of this essay it was
pointed out that the causes to which I attributed
the reversion of the hypothetical Amblystoma
Mexicanum to the existing Axolotl, did not appear
to me to amount to a complete explanation
of the phenomenon. In the first place these
seemed to me too local, since they could only be
applied with any certainty to the Axolotl of the
lake of the Mexican capital, whilst the Paris
Axolotls obtained from other parts of Mexico
still required an explanation. On the other hand,
these causes did not appear to me sufficiently
cogent. Should we even learn subsequently that
the Paris Axolotl is also derived from a salt lake
which is exposed to similar winds to the Lake of
Mexico, we still have in this peculiarity of the
lakes only a cause tending to make it difficult for
the larva to undergo metamorphosis, and to reach
a suitable new habitat on the land. The impossibility
of doing this, or the complete absence of
such habitat, does not however follow as a necessary
consequence.
It would obviously be a much more solid support
for my hypothesis if it were possible to point
to some physical conditions of the land which
there precluded the possibility of the existence of
Amblystomas.
For a long time I was indeed unable to discover
such causes, and I therefore concluded the previous
portion of this essay and went to press.
Afterwards, when residing in one of the highest
valleys of our Alps in the Upper Engadine, an
idea accidentally occurred to me, which I do not
now hesitate to regard as correct after having
tested it by known facts.
It happens that in the Upper Engadine there
live only such Amphibia as persistently, or at
least frequently resort to the water. I found
frogs up to nearly 7000 feet above the sea, and
Tritons at 6000 feet (Pontresina and Upper
Samaden). On the other hand, the land-living
mountain salamander, S. Atra,267 was absent, although
suitable stations for this species were
everywhere present, and it would have wanted for
food as little as do its allies the water-newts.
Neither would the great elevation above the sea
offer any obstacle to its occurrence, since it occasionally
ascends to a height of 3000 metres
(Fatiot).268
Now it is well known that the atmosphere of
the Upper Engadine,269 like that of other elevated
Alpine valleys enclosed by extensive glaciers, is
often extraordinarily dry for a long period, a condition
which appears to me to explain why the
black land-salamander is there absent,270 whilst its
near water-living ally occurs in large numbers.
The skin of the naked Amphibia generally requires
moisture, or else it dries up, and the creature is
deprived of a necessary breathing apparatus, and
often dies as rapidly as though some important
internal organ had been removed. Decapitated
frogs hop about for a long time, but a frog which
escapes from a conservatory and wanders about
for one night in the dry air of a room, is found the
following day with dry and dusty skin half dead in
some nook, and perhaps perishes in the course of
another day if left without moisture.
All that we know of the biology of the Amphibia
is in accordance with this. Thus, all the land-salamanders
of southern Italy avoid the hot and
dry air of summer by burying in the ground, where
they undergo a summer sleep. This is the case
with the interesting Salamandrina Perspicillata,271
and with the land-living Sardinian Triton, the
remarkable Euproctus Rusconii, Gené,272 (Triton
Platycephalus, Schreiber). With respect to Geotriton
Fuscus I learn from Dr. Wiedersheim, who
has studied the life conditions of this, the lowest
European Urodelan, in its own habitat, that in
Sardinia it sleeps uninterruptedly from June till
the winter; whilst on the coast of Spezia and at
Carrara, where it also occurs, it avoids the summer
sleep in a very peculiar manner. It makes use of
the numerous holes in the calcareous formation of
that region, and for some months in the year
becomes a cave-dweller. As soon as the great
heat occurs, often in May, it withdraws into the
holes, and again emerges in November during the
wet weather. In these lurking holes it does not
fall into a sleep, but is found quite active, and
its stomach, filled chiefly with scorpions, shows
that it goes successfully in search of food; the
moist air of the holes makes it unnecessary for it
to bury in the earth.
In the same sense it appears to me must be
conceived the fact that the solitary species of frog
of the Upper Engadine, Rana Temporaria,273 the
brown grass frog, is there much more a frequenter
of the water than in the plains. It is true that I
can find no remark to this effect in the excellent
work of Fatiot, already referred to above, and I
am therefore obliged to resort to my own observations,
which, although often repeated, have always
been carried on for only a short time. I was
much struck with the circumstance that the Engadine
frogs were to be found in numbers in the
water long after the pairing season, which, according
to Fatiot, lasts at most to the end of June.
In the numerous pools around Samaden I found
them in July and August, whilst in the plains they
only take to the water at the time of reproduction,
and seek winter quarters in the mud on the first
arrival of this season. (Fatiot, p. 321.) In the
Engadine they have therefore in some measure
adopted the mode of life of the aquatic frogs, but
this of course does not prevent them from returning
in damp weather to their old habits and
roving through meadows and woods.
After these considerations had made it appear
to me very probable that the dry air of the Upper
Engadine accounted for the absence of the black
land-salamander, the question at once arose
whether the absence of Amblystomas from the
Mexican plateau might not perhaps be due to the
same cause, i.e. whether such a dryness of the
atmosphere might not perhaps prevail also in that
region, so that Amphibia, or at least salamander-like
Amphibia, could not long exist on the land.
The height above the sea is still greater (7000
to 8000 feet), and the tropical sun would more
rapidly dessicate everything in a country poor in
water.
As I was at the time without any books that
might have enlightened me on the meteorological
conditions of Mexico, I wrote to Dr. v. Frantzius,
who, by many years residence in Central America
was familiar with the climate of this region, and
solicited his opinion. I received the reply that
on the high plains of Mexico an extraordinary
dryness of the atmosphere certainly prevails.
“The main cause of the dryness of the high
plains is to be found in the geographical position,
the configuration of the land, and the
physical structure. The north-eastern trade-wind
drives the clouds against the mountains,
on the summits of which they deposit their
moisture, so that no vapour is carried over;
as long as the north-east trade-wind blows, the
streams feeding the rivers flowing into the Atlantic
Ocean are abundantly fed with water, whilst
on the western slopes, and especially on the high
plains, the clouds give no precipitation. In the
second half of the year also, during our summer,
the so-called rainy season brings but little rain274—little
in comparison with the more southern regions,
where the heavy tropical thunderstorms daily
deluge the earth with water. Mexico lies much
too northerly, and does not reach the zone of
calms, within which region these tropical rains
are met with.”
Thus, in the high degree of dryness of the air
lasting throughout the year, I do not doubt that
we have the chief cause why no Amblystomas
occur on these elevated plains; they simply cannot
exist, and would become dried up if taken there,
supposing them not to be able to change their
mode of life and to take to the water. If therefore
in former times Amblystomas inhabited
Mexico, the coming on of the existing climatic
conditions left them only the alternative of becoming
extinct, or of again taking to the aquatic
life of their Ichthyodeous ancestors. That this
was not directly possible—that the Amblystoma
form was not able to become aquatic without a
change of structure, is shown by the fact that
even in the Lake of Mexico no Amblystoma
occurs. A retreat to an aqueous existence could,
as it appears, only be effected by complete reversion
to the Ichthyodeous form, which then also
took place.
But my hypothesis of the transformation of the
Axolotl not only requires the proof that Amblystomas
cannot exist under present conditions in
Mexico, but also the further demonstration that at
a former period other conditions prevailed there,
and these of such a nature as to make the
existence of land-salamanders possible.
With respect to my question, whether we might
not perhaps assume that at some post-glacial
period the conditions of atmospheric moisture on
the high plains of Mexico were essentially different
from those at present prevailing, I recollected Dr.
v. Frantzius and the above-quoted observation of
Humboldt’s,275 who discovered in the neighbourhood
of the Lake of Tezenco (Mexico) distinct
evidence of a much higher former level of the
water. “All such elevated plains were certainly
at a former period so many extensive water-basins,
which gradually became filled, and are still filling
up with detritus. The evaporation from such
large surfaces of water must at that time have
caused a very moist atmosphere, favourable to
vegetation and adapted for the life of naked
Amphibia.”
From this side also my hypothesis thus receives
support, and we may assume with some certainty
that at the beginning of the diluvial period276 the
woods surrounding the Mexican lakes were inhabited
by Amblystomas, which, as the lakes subsequently
became more and more dried up and the
air continually lost moisture, found it more difficult
to exist on the land. They would at length
have completely died out, had they not again
become aquatic by reversion to the Ichthyodeous
form. It may perhaps be supposed that the
above-mentioned physical conditions—desolate,
salt-incrusted shores—co-operated in the production
of the reversion, by making it difficult for
the larvæ to quit the water; but we can only judge
with certainty upon this point when, by means of
experiment, we have discovered the causes which
produce reversion in the Amphibia.
I have lately met with another interesting
notice on the reproduction of the native North
American Amblystomas. Professor Spence F.
Baird, of Washington, has often observed the
development from the egg of various species, and
especially of Amblystoma Punctatum and A. Fasciatum.
His observations do not appear to be as
yet published, so that I was unable to discover
any account of the development of Amblystoma in
existing literature.277 I am authorized to extract
the following brief data from a letter addressed to
Dr. v. Frantzius.
In order to deposit their eggs the Amblystomas
go into the water, where the eggs are laid enclosed
in a jelly-like mass, but never more than fifteen to
twenty together. The spherical eggs are very
large, perhaps a quarter of an inch in diameter.
They soon develop into a Siredon-like larva, which
remains several months in this condition. The
gills then shrivel up, the creature begins to crawl,
and gradually passes through the different transformations
to the complete Amblystoma form.
It appears from this communication that the
Amblystomas lay much larger and much fewer
eggs than the Axolotl, and that their development
throughout resembles that of our salamanders.
In concluding I may mention an anatomical
fact which most strongly supports my view that
the Mexican Axolotl is a reverted Amblystoma.
I learn from Dr. Wiedersheim that the Axolotl
possesses the “intermaxillary gland” which occurs
in all the land Amphibia. This organ, lying in the
intermaxillary cavity, appears, whenever it occurs,
to produce a kind of birdlime, i.e. a very glutinous
secretion, which serves to attach the prey to
the rapidly protrusible tongue. Although this
secretion may perhaps also have another function,
from the absence of the intermaxillary gland in all
exclusively aquatic Amphibia, it follows that it
must be devoid of importance for, and inapplicable
to feeding in the water. The intermaxillary gland
is absent in all Perennibranchiata and Derotremata
which Wiedersheim has hitherto investigated,
viz. in Menobranchus, Proteus, Siren,
Cryptobranchus, Amphiuma, and Menopoma, all
of which are indeed without the cavity in which
the gland is situated in the Salamandrina, i.e.
the cavum intermaxillare.
Now in the Salamandrina the gland appears
at an early stage. It is possessed in a well-developed
state by the larvæ both of species of
Triton and of Amblystoma, where indeed the
glandular structure completely fills the cavum
intermaxillare.
Were the Axolotl a species retarded in phyletic
development, the presence of a gland which does
not occur in any other Perennibranchiata, and
which is only of use for life upon land, would be
quite inexplicable.
The matter becomes still more enigmatical
through the fact that the gland, although present,
is quite rudimentary. Whilst in the Salamandrina
the capacious intermaxillary cavity is entirely
filled by the tubes of the gland in question,
in Axolotl this cavity is almost completely filled
with a closely woven connective tissue, in which
there can only be found a small number of gland-tubes—in
the extreme front, and at the base immediately
over the intermaxillary teeth—these
tubes agreeing in the details of their histological
structure with the elements of the same gland
in the Salamandridæ.
I give these anatomical details from Dr.
Wiedersheim’s verbal communication. An amplified
account will subsequently appear in another
place.278
An explanation of this rudimentary intermaxillary
gland in the Axolotl only appears to me possible
on the supposition that the latter is an atavistic
form. From this point of view it is evident that
the gland already present in all Amblystoma-larvæ
must have been taken over by the perennibranchiate
form of the existing Axolotl, through
the reversion of the hypothetical Amblystoma
Mexicanum of the “diluvial period.”279 It can
also be easily understood that this organ would
become more and more rudimentary in the course
of time, since it has no further use in the water,
and the gap thus arising in the formerly present
cavum intermaxillare would become filled with
connective tissue.
While the German edition of this work was
going through the press I obtained, through the
kindness of my friend Dr. Emil Bessels of
Washington, the Mexican memoir upon the new
Axolotl,280 which even in Mexico regularly, or at
least in many cases, becomes developed into the
Amblystoma form.
The facts are briefly as follows:—The small
Lake of Santa Isabel is some hours’ journey from
the Mexican capital. In this lake there lives a
species of Axolotl which had hitherto remained
unknown, and was described by Señor Velasco as
Siredon Tigrinus. This species propagates itself
indeed in the Axolotl state, but in many cases it
becomes transformed into Amblystoma and takes
to the land. Although propagation in the Amblystoma
condition was not observed, it can hardly be
doubted that it also propagates in this form.
At first sight these facts appear to refute my
hypothesis, that the extreme dryness of the air of
the Mexican plateau precludes the existence of
land Amphibia. Nevertheless I do not abandon
this hypothesis for the former one, since a closer
study of the data furnished by Velasco confirms
rather than refutes my supposition.
Velasco expressly corroborates the statement
that the Axolotl hitherto known from the great
Mexican lake which never dries up (Lake of
Xochimilco and Chalco), is only met with in its
native habitat in the Siredon form, i.e. as Siredon
Humboldtii. According to Velasco the cause of
the frequent assumption of the Amblystoma form
by the new Siredon Tigrinus, is to be found in the
local conditions of life of this species. The Lake
Santa Isabel is shallow, its greatest depth amounting
to three meters, and it is liable to a periodical
drying up, which is so complete that one can pass
dry-shod through it in several places. The species
must therefore have long since died out had it
not been able to adapt itself periodically to a
land life. Now it could have become transformed
into a land Amphibian—as Señor Velasco observed—at
various stages of growth; and indeed
this author believes that “the Creator has implanted
an instinct in this creature,” which enables
it to always undergo metamorphosis at the right
time.
This last assumption may or may not be taken
as correct, but this much is established, viz. that
numerous individuals of this species take to the
land, and remain there during a period of many
months.
But does this contain the proof that salamander-like
animals are actually able to lead a land life in
Mexico—that the dry air is advantageous, or at
least supportable to them? It does not appear so
to me, but rather that all which has been reported
of this Amblystoma by Señor Velasco goes to
show that the animal does not, properly speaking,
live upon land like the North American Amblystomas,
or like our land-salamanders, but that it
only experiences a summer sleep lasting over the
period of drought. These Amblystomas were
observed as they left the dried-up lake at night in
order to seek some moist lurking-place in the
neighbourhood, where they might remain concealed.
They are only known in the villages
situated near the lake, and were only seen there at
large just when they were wandering from the lake
to their place of concealment. At other times
they were mostly found in the earth, buried under
walls, the pavement of the market-place, &c.
When laying down a line of railway, a workman
found in the earth a whole nest of twelve Amblystomas
lying close together. All these are not
mere lurking-holes which could be abandoned at
any moment; it would rather appear that we have
here places of refuge for the entire duration of the
period of drought, and that these would only be
forsaken when the water of the rainy season penetrated
the soil. I am not myself in a favourable
position for investigating these suppositions more
closely, but this could be done by Señor Velasco,
who lives in Mexico, and science would be much
indebted to him if he would examine as precisely
as possible into the habits and conditions of life of
this, and of the other species of Mexican Axolotls.
Unfortunately this gentleman can, it would appear,
have seen only the French publications upon
the transformation of the Axolotl, and could not
therefore have asked himself questions arising
from my conception of the facts; otherwise many
of his observations would have led to more definite
results. The above conclusion can however be
still further supported by Señor Velasco’s data.
One might indeed insist that with us also the
land-salamanders conceal themselves in moist
places during dry weather, and often lie hidden,
as in Mexico, in a hole, in a cluster of as many as
ten together; but with us they leave their lurking-place
from time to time and go in search of food.
Señor Velasco mentions nothing with respect to
this. What especially struck me was the statement
that the Mexican Amblystomas were also to
be found in the water.281 When Lake Santa Isabel
is drained, the fishermen stretch large nets across
the exit channels, and in these they not only find
ordinary Axolotls, but also some “sin aretes,”
which they also designate “mochos,” i.e. hornless
Axolotls, because they have no gills, but have
already reached the Amblystoma stage. Our
land-salamanders live in the water only as larvæ,
but they also love and require moisture. Only the
female enters the water when she wants to deposit
her young (eggs with mature larvæ), and then only
at the margin of shallow pools or small brooks.
The Mexican Amblystoma thus much more resembles
in its habits our water-salamanders
(Tritons), which remain in the water at least during
the whole period of reproduction. These also
leave the water later, and, like the land-salamander,
seek concealment in the earth. They have this
habit also in those districts which possess a very
dry atmosphere; and especially in the Engadine,
where I first conceived the idea of taking into
account the dryness of the air, I found in the
pools at the end of August and the beginning of
September only larvæ of Tritons. The older
Amphibians must therefore have been on the land,
presumably in their places of winter concealment.
From what we have hitherto learnt from Señor
Velasco, the mode of life of Amblystoma Tigrinum
must resemble that of our Tritons, although its
structure is that of a land-salamander. I would
thus offer the following explanation of the facts at
present known:—Owing to the periodic drying up
of the lake of Santa Isabel, the Siredon Tigrinus
would be again compelled to undergo metamorphosis.
Whether this was formerly entirely
abandoned, or whether it always occurred in solitary
individuals, is almost immaterial; in any case the
habit of metamorphosis must have been very
rapidly acquired through natural selection, and
must have again become general, if the faculty
was only present in the species, although latent.
Through the dryness of the air, the Amblystomas
that had taken to the land would be compelled to
bury themselves at once, and to remain asleep till
the recurrence of the rainy season, when they
would hasten back into the water and would there
live as a species of Triton.
Now one might feel inclined to ask why the
species of the great Mexican lake has not also
taken to this mode of life. To this it may be
simply replied that the water of this lake never
dries up, and that the Axolotls have thus never
been reduced to the alternative of undergoing
metamorphosis or of perishing. If therefore the
conditions of existence in water were more favourable
than on land, the tendency to abandon metamorphosis
would increase from generation to
generation, and the deportment at present observed
would finally result, i.e. propagation would
take place exclusively in the Axolotl state. As
has already been mentioned above, the latest
observations of Velasco furnish further confirmation
that the Axolotl of the great lake is never
met with in the Amblystoma condition, “although
it (the Axolotl) is brought daily from Mexico
into the market throughout the whole year.” I
should not however regard it as a refutation of
my view if prolonged investigation should show
that this species also (Siredon Humboldtii) occasionally
developed into an Amblystoma; on the
contrary, it would not at all surprise me if such
cases of reversion occurred in Mexico as well as
in Europe. The fact that an immense majority of
the Amphibians propagate in the Axolotl state
would not be thereby affected, and would still
require an explanation: this I am still inclined to
see in the dryness of the air of the high plains,
which is so unfavourably adapted for a life passed
entirely on land.
