ith 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
purpose has been to show how the phenomena of development are viewed by
men of science, and how they take their place in the doctrine of organic
evolution. And it has also been made plain that comparative anatomy and
comparative embryology support and supplement one another in countless
ways and places, although each in itself is a complete demonstration that
evolution is a real and a natural process.

III

THE EVIDENCE OF FOSSIL REMAINS

Few natural objects appeal to the interest and imagination of the student
with more force than the fragments of animals and plants released from the
rocks where they have been entombed for ages. Our lives are so brief that
it is impossible for us to comprehend the full duration of the slow
process which constructed the burial shrouds of these creatures of long
ago. We try to picture the earth and its inhabitants as they were when
lizards were the highest forms of animals, and we wonder how life was
lived in the dense forests of the coal age. Science can never learn all
about the ancient history of the earth and of the organisms of bygone
times; yet it has been able to accomplish much through its endeavors to
reconstruct the past, for its method is one by which sure results can
always be obtained whenever there are definite facts with which it can
work. In our present study of evolution we reach the point when we must
examine the testimony of the rocks, and the res