escent which have led up to them and
which are comparable to the branches and limbs arising from the trunk of a
tree. Thus the major and minor divisions of animals do not follow in the
order of the rungs of a ladder, even though they must be assigned to
different levels according to the complexity of their construction. The
summary given above, namely, that the occurrence of lower and higher
levels reveals an order of evolution, is amplified and not contradicted by
the statement that the species of animals are group in a treelike
arrangement. It is the task of the evolutionist, provided with all the
facts of comparative anatomy and dealing only with the various species as
separate leaves, so to speak, to reconstruct the now invisible but not
unreal twigs and branches and limbs of the animal tree, and to show how
they have diverged at one time or another as they have grown and spread to
produce the species of the present day. This he may do in so far as he may
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 ac