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 Filippi[244] 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 Forel[250] 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.
Darwin[257] 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 Mexico[259] 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, Schreibers[263] 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 Humboldt[265] 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]