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The Eighth Lesson: The Ascent of Man.827
while in the water, and also have a primitive or elementary “lung” in the
shape of an air-bladder, or “sound,” which they use for breathing on land.
The Mud-fish of South America, and also other forms in Australia and other
places, have a modification of fins which are practically “limbs,” which they
actually use for traveling on land from pond to pond. Some of these fish
have been known to travel enormous distances in search of new pools of
water, or new streams, having been driven from their original homes by
droughts, or perhaps by instincts similar to the migrating instinct of birds.
Eels are fish (although many commonly forget this fact) and many of their
species are able to leave the water and travel on land from pond to pond,
their breathing being performed by a peculiar modification of the gills. The
climbing perch of India are able to live out of water, and have modified gills
for breathing purposes, and modified fins for climbing and walking. So you
see that without leaving the fish family proper, we have examples of land
living creatures which are akin to “connecting links.”
But there are real “connecting-links”’ between the Fish and the Reptiles.
Passing over the many queer forms which serve as links between the two
families, we have but to consider our common frog’s history for a striking
example. The Tadpole has gills, has no limbs, uses its tail like a fish’s fin, eats
plants, etc. Passing through several interesting stages the Tadpole reaches a
stage in which it is a frog with a tail—then it sheds its tail and is a full fledged
Frog, with four legs; web-feet; no tail; and feeding on animals. The Frog is
amphibious, that is, able to live on land or in water—and yet it is compelled
to come to the surface of the water for air to supply its lungs. Some of the
amphibious animals possess both lungs and gills, even when matured; but
the higher vertebrates living in the water breathe through lungs which are
evolved from the air-bladder of fishes, which in turn have been evolved
from the primitive gullet of the lower forms. There are fishes known which
are warm-blooded. Students will kindly remember that the Whale is not a
fish, but an aquatic animal—a mammal, in fact, bringing forth its young alive,
and suckling it from its breasts.