find sufficient materials to enable him to employ the methods of
comparative anatomy and the great natural principle established by this
method--that essential likeness means consanguinity.

* * * * *

No evidence of evolution could be more significant and interesting than
the results provided by the comparative study of development. In the first
place it is an obvious fact that every living thing changes in the course
of its life-history, and if as an adult it occupies a high place in the
animal scale, its embryological transformation is more elaborate and
intricate than in the case of a lower form. Every one knows that organisms
do develop, and yet I believe that few appreciate the tremendous
significance of the mere fact that this is true, while still fewer are
aware that the peculiar and characteristic early stages through which an
animal passes in becoming an adult are even more striking than the fact of
development itself. We shall learn something of these earlier conditions
in the development of some of our most familiar animals, but at the outset
nothing can be more important than an appreciation of the first great
lesson of this department of natural history--namely that organic
transformation is real and natural. We do not need to employ the methods
of formal logic to know that in growing up a human infant undergoes the
changes of childhood and adolescence, that kittens become cats, and that
an oak tree is produced by an acorn, 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