ryology has been
particularly valuable in just that part of the history where the
evidence from the study of fossils is wanting. The study of fossils, as
we have seen, gives little or no data concerning the early history of
living machines; and it is just here that embryology has proved to be of
the most value. It is a source of evidence that has told us of most of
the steps in the progress from the single-celled animal to the
multicellular organisms, and gives us the clearest idea of the
fundamental principles which have been concerned in the evolution of
life and the construction of the complicated machine out of the simple
bit of protoplasm. In spite of its limits, therefore, embryology has
contributed a large quota of the evidence which we have of the evolution
of life.

==Anatomical.==--A third source of this history is obtained from the facts
of comparative anatomy. The essential feature of this subject is the
fact that animals and plants show relationships. This fact is one of the
most patent and yet one of the most suggestive facts of biology. It has
been recognized from the very beginning of the study of animals and
plants. One cannot be even the most superficial observer without seeing
that certain forms show great likeness to each other while others are
much more unlike. The grouping of animals and plants into orders,
genera, and species is dependent upon this relationship. If two forms
are alike in everything except some slight detail, they are commonly
placed in the same genus but in different species, while if they show a
greater unlikeness they may be placed in separate genera. By thus
grouping together forms according to their resemblance the animal and
vegetable kingdoms are classified into groups subordinate to groups. The
principle of relationship, i.e., fundamental similarity of structure,
runs through the whole animal and vegetable kingdom. Even the animals
most unlike each other show certain points of similarity which indicates
a relationship, although of course a distant one.

The fact of such a relationship is too patent to demand more words, but
its significance needs to be pointed out. When we speak of relationship
among men we always mean historical connection. Two brothers are closely
related because they have sprung from common parents, while two cousins
are less closely related because their common point of origin was
farther back in time. More widely we speak of the relationship of the
I