ave been so greatly modified, how can we make
even a shrewd guess at the ancestry of higher forms? The genealogy
of the animal kingdom has been really the study of centuries,
although the earlier zooelogists did not know that this was to be the
result of their labors. The first work of the naturalist was
necessarily to classify the plants and animals which he found, and
catalogue and tabulate them so that they might be easily recognized,
and that later discovered forms might readily find a place in the
system. Hypotheses and theories were looked upon with suspicion.
"Even Linnaeus," says Romanes, "was express in his limitations of
true scientific work in natural history to the collecting and
arranging of species of plants and animals." The question, "What is
it?" came first; then, "How did it come to be what it is?" We are
just awakening to the question, "Why this progressive system of
forms, and what does it all mean?"

Let us experiment a little in forming our own classification of a
few vertebrates. We see a bat flying through the air. We mistake it
for a bird. But a glance at it shows that it is a mammal. It is
covered with hair. It has fore and hind legs. Its wings are
membranes stretched between the fingers and along the sides of the
body. It has teeth. It suckles its young. In all these respects it
differs from birds. It differs from mammals only in its wings. But
we remember that flying squirrels have a membrane stretching along
the sides of the body and serving as a parachute, though not as
wings. We naturally consider the wings as a sort of after-thought
superinduced on the mammalian structure. We do not hesitate to call
it a mammal.

The whale makes us more trouble; it certainly looks remarkably like
a fish. But the fin of its tail is horizontal, not vertical. Its
front flippers differ altogether from the corresponding fins of
fish; their bones are the same as those occurring in the forelegs of
mammals, only shorter and more crowded together. Later we find that
it has lungs, and a heart with four chambers instead of only two, as
in fish. The vertebrae of its backbone are not biconcave, but flat in
front and behind. And, finally, we discover that it suckles its
young. It, too, is in all its deep-seated characteristics a mammal.
It is fish-like only in characteristics which it might easily have
acquired in adaptation to its aquatic life. And there are other
aquatic mammals, like the seals, in which these cha