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.
=IV.=