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 rain[274]--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
period[276] 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.