Returning to our pond this morning to see whether the water-lilies
propose flowering this season, I find that the frogs have been
depositing spawn along its edges, so that the thongs of Irish leather
may become necessary sooner than I expected!
All the same I am delighted to see the frog-spawn, for I have an
affection for tadpoles. Youthful associations cluster pretty thickly
around them, but apart from such a merely sentimental attachment, there
is a satisfaction, I may say a zoologic thrill, about this transition of
a water-living and water-breathing animal into an air-breathing one; a
transition going on, moreover, not at some remote, and more or less
dubious geologic age, but under one’s very eyes, even, as in this case,
in the middle of one’s own decorous, shaven lawn.
It is difficult to remember that frogs breathe air as much as we do
ourselves. Unlike ourselves, and their other zoologic betters, they do
so, however, not by alternate contractions and dilations of the chest,
Nature not having provided them with ribs, but by the doubtless more
archaic process of swallowing air. Not only would a frog die if kept too
long under water, but--seeing that it can only swallow air by shutting
its mouth--were that mouth kept forcibly open it would equally die, and
from the same cause, namely, want of breath. Tadpoles, on the other
hand, are strictly water-breathers, and until they have shed their
gills, have no more need to go to the surface to breathe than a fish
has. That, by the way is not an absolutely accurate illustration, seeing
that certain fishes _do_ need to go to the surface for air. The famous
Anabas, or “climbing perch” of India, is such an air-breathing fish, the
air reaching it by means of cavities on either side of its gills, and if
prevented from reaching the surface, and renewing the supply, it would
“drown like a dog,” or so the scientists assert. Such cases, however,
can hardly be called normal. Fishes that can live comfortably for days
out of the water, that can nest in a bush, and travel across a
particularly dry country, are not likely to be met with in zoologic
rambles about this parish.
Returning to our Irish frogs, it is an odd fact, especially considering
their recent introduction, that in addition to swarming over the
lowlands, and in every place dear to frogs, they have learnt to climb
long distances up hill, and to establish themselves in ponds separated
widely from any others, often not even fed by streams, and moreover
destitute of nearly all other animal inhabitants, with the exception of
certain minute molluscs, which are believed by zoologists to have
reached them upon the feet of wading birds, and that at such a remote
period of time that they have become what are practically new species.
Many years ago, on reaching the top of Mweelrea, the leading mountain of
Connemara, I remember my surprise at finding swarms of young tadpoles
wriggling along the margin of a small pond, nearly upon the actual
summit. They were still in the engaging comma-like stage, before legs
had begun to dawn upon their consciousness, and seemed to have
remarkably little to eat, for the water was crystal clear. The pond was
one of that attractive kind known as _corries_, held by the geologists,
doubtless truly, to be of glacial origin; a delicious clean-cut oval;
pure rock, from marge to marge; gouged, as if by the chisel of Michael
Angelo, from the matrix in which it lay. But for the unmistakable
evidence of the tadpoles it would, to any reasonable imagination, have
suggested the bath of some mountain nymph very much sooner than
frog-spawn.
We are all of us to-day evolutionists, if some of us still with a
certain amount of reservation, and to the evolutionist tadpoles must
always prove interesting acquaintances. They provide us with at least an
inkling as to the fashion in which your unadulterated water-breather may
have been converted into an air-breather, and by means of no process
more recondite than that of losing its gills. That such conversions do
take place, and under certain circumstances remain permanent, has been
proved in the well-known case of the axolotl, or Mexican gilled
salamander. As long ago as the year 1867, while conducting some
experiments at the Jardin des Plantes, M. Duméril startled the zoologic
world of Paris by communicating the fact that, out of a number of
axolotls kept in the collection there, about thirty had left the water,
and had assumed the form of what had hitherto been regarded as an
absolutely distinct genus of land salamander, known as amplystoma. This
discovery made at the time a prodigious stir, not so much on account of
a water-breathing creature losing its gills, and becoming an
air-breather, for that was a phenomenon which might be seen every
spring, and in most of the ditches round Paris, but because the axolotl
was known to breed, and that it therefore appeared to indicate the
exceedingly anomalous case of a larval form proving to be fertile.
How the feat of transformation was to be actually witnessed was the next
problem, and it is pleasant to remember that it was through the energy
and perseverance of a woman naturalist, Fraulein Marie von Chauvin, that
the matter was finally cleared up. By continually damping the specimens
of axolotl kept by her on land, and assiduously feeding them, she was
able to preserve two out of five through the gradual process of
decreasing their gill-tufts, and tail-fins, changing their skins, and so
forth. Finally to her own and everyone’s triumph, the complete
amplystoma form was assumed, and the transformation was thereby
accomplished. The world has seen a fair number of miracles since it
began to run its course, and perhaps not the least difficult of those
miracles to receive with absolute credulity have been some of its
natural ones!