y "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.

So we readily see that it is but a step, and a short step at that,
between the land-traveling and climbing fishes and the lower forms of
Reptiles. The Frog shows us the process of evolution between the two
families, its life history reproducing the gradual evolution which may
have required ages to perfect in the case of the species. You will
remember that the embryo stages of all creatures reproduce the various
stages of evolution through which the