, life, as thus viewed, seems to
have its root in chemical change; and we can understand how an eminent
German physiologist offers us a definition or characterization of life
that runs: "The life-process consists in the metabolism of proteids."
I ask your particular attention to this definition since I now wish to
contrast with it another and very different one.

I shall introduce it to your attention by asking a very simple
question. We may admit that digestion, for example, is a purely
chemical operation, and one that may be exactly imitated outside the
living body in a glass flask. My question is, how does it come to pass
that an animal has a stomach?--and, pursuing the inquiry, how does it
happen that the human stomach is practically incapable of digesting
cellulose, while the stomachs of some lower animals, such as the goat,
readily digest this substance? The earlier naturalists, such as
Linnaeus, Cuvier or Agassiz, were ready with a reply which seemed so
simple, adequate and final that the plodding modern naturalist cannot
repress a feeling of envy. In their view plants and animals are made
as they were originally created, each according to its kind. The
biologist of to-day views the matter differently; and I shall give his
answer in the form in which I now and then make it to a student who
may chance to ask why an insect has six legs and a spider eight, or
why a yellowbird is yellow and a bluebird blue. The answer is: "For
the same reason that the elephant has a trunk." I trust that a certain
rugged pedagogical virtue in this reply may atone for its lack of
elegance. The elephant has a trunk, as the insect has six legs, for
the reason that such is the specific nature of the animal; and we may
assert with a degree of probability that amounts to practical
certainty that this specific nature is the outcome of a definite
evolutionary process, the nature and causes of which it is our
tremendous task to determine to such extent as we may be able. But
this does not yet touch the most essential side of the problem. What
is most significant is that the clumsy, short-necked elephant has been
endowed--"by nature," as we say--with precisely such an organ, the
trunk, as he needs to compensate for his lack of flexibility and
agility in other respects. If we are asked _why_ the elephant has a
trunk, we must answer because the animal needs it. But does such a
reply in itself explain the fact? Evidently not. The question which
s