orn, for we know these things directly by
observing them. It is natural for development to take place under normal
conditions, and if it does not, then something has interfered with nature.
Inasmuch as "growing up" is accomplished by the alteration of an organic
mechanism with one structure into an individual with a changed plan of
body, it is in essence the actual process of evolution which the
comparative study of grown animals of to-day demonstrates in the way we
have learned. The study of animal structure discovers the process of
evolution because the most reasonable interpretation of the similarities
and minor differences exhibited everywhere by the various groups of
animals is that descent with adaptive and divergent modification has taken
place; the result is reached by inference, it is true, but by scientific
and logical inference. With development it is otherwise. No reasoning is
necessary to tell us that organic transformation is a real and a natural
process. We see it everywhere about us and we ourselves have come to be
what we are by a natural history of change. Can we consistently deny that
it is possible for a species to alter in the long course of time when a
few brief weeks are sufficient for the new-laid egg of the fowl to develop
into a fledgling? Many indeed strain at the gnat of the longer process in
the past when without hesitation they recognize the real and obvious fact
of individual development in a brief period.

I have said that development is a "natural" process. We employ this word
for the familiar and everyday occurrence or thing; it does not imply that
everything is known about the object or phenomenon, because science knows
that complete and final knowledge is impossible. We say that it is natural
for rain to fall to the earth, and we speak of the law of gravitation
according to which this takes place as a natural principle, but it may not
have occurred to many to inquire _what_ makes rain fall and _why_ do
masses of matter everywhere behave toward one another in the consistent
manner described by the law in question. Sunshine is natural, but we do
not know _why_ light travels as it does from the sun to the earth, and
this is another question which, like the inquiry into the ultimate cause
of the familiar and natural phenomenon of gravitation, has not yet been
answered. But it is still regarded as natural for the rain to fall and for
the sun to shine. In the same way does science view develo