The basic characteristics of every group in a high position may be traced
back to some one or another of the divisions at a lower level, so that the
general sequence of the structural levels from low to high becomes
intelligible as the order of their evolution.

To my mind the rudimentary and vestigial structures of animals are in
themselves proof positive of a natural history of change. The few
illustrations can be reinforced by countless examples offered by every
group of living animals. If such structures have not evolved naturally by
degenerating from more efficient counterparts in ancestors of earlier
times, and if they have been specially created, they are utterly
meaningless and their very existence is unreasonable. If common sense is
to be employed, they demonstrate evolution.

Everywhere throughout the whole series animals place themselves in a
treelike arrangement, for in their respective levels they occur like
leaves at the ends of the lines of descent 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