the same method which in comparative anatomy finds that consanguinity is
expressed by resemblance. The great law of recapitulation, stated in one
form by Von Baer and more definitely by Haeckel in the terms employed in
the foregoing sections, was for a time too freely used and too rigidly
applied by naturalists whose enthusiasm clouded their judgment. A strong
reaction set in during the latter part of the nineteenth century, when
attention was directed to the anachronisms of the embryonic record and to
the alterations that are the results of larval or embryonic adaptation as
short cuts in development. Nevertheless, it is not seriously questioned, I
believe, that the main facts of a single life-history owe their nature to
the past evolution of the species to which a given animal belongs.

Nowadays the problems in this well-organized department are concerned not
only with more accurate accounts of the development of animals, but also
with the mechanics of development, with the relative value of external and
internal influences, and above all with the physical basis of inheritance.
It is clear that the factors that direct the development of a wood frog's
egg so that it becomes a wood-frog and not a tree-toad must lie in the egg
itself, as derivatives from the two parent organisms. Weismann and his
followers have proved that a peculiar substance in the nuclei of the egg
and its daughter-products contains the essential factors of development,
whatever these may be. Experiments dealing with the phenomena of heredity
in pure and mixed breeds have largely confirmed Weismann's doctrine, and
they have prepared the way for a deeper investigation of the marvelous
process of biological inheritance.

However much he may be interested in the details of embryological science,
the general student of natural history is more concerned with the bearing
of its primary laws upon the great problem of evolution. In the foregoing
brief review of the fundamental facts and principles of this subject, the