“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 observed[228] 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
work[230] 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 Marsh[231] 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 zoologists[236] 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 follows[237]:--“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).