y, more and more characters are found which are the common
properties of wider and wider arrays of organisms, for at one time the
embryo exhibits gill-slits in the sides of its throat which in all
essential respects are just like those of the embryos of birds and
reptiles and amphibia, as well as of other embryo mammals and these
gill-slits are furthermore like those of the fishes which use them
throughout life. All the other organic systems exhibit everywhere the
common characteristics in which the embryos of the so-called higher animals
agree with one another and with the adult forms among lower creatures; the
human embryo possesses a fishlike heart and brain and primitive backbone,
fishlike muscles and alimentary tract. Can we reasonably regard these
resemblances as indications of anything else but a community of ancestry
of the forms that exhibit them?

Yet a still more wonderful fact is revealed by the study of the very
earliest stages of individual development. The human embryo begins its
very existence as a single cell,--nothing more and nothing less; in
general structure the human egg, like the eggs of all other many-celled
organisms, is just one of the unitary building blocks of the entire
organic world. And yet the egg may ultimately become the adult man. Does
this mean that man and all the other higher forms have evolved from
protozoa in the course of long ages? Science asks if it can mean anything
else. When the comparative anatomist bids us look upon the wide and varied
series of adult animals lower than man as his relatives, because they
display similar structural plans beneath their minor differences, it may
be difficult at first to obey him. But in the brief time necessary for the
human egg to develop into an adult, the entire range is compassed from the
single cell to the highest adult we know. There are no breaks in the
series of embryonic stages like those between the diverse adult animals of
the comparative array. I do not think we could ask nature for more
complete proof that human beings have evolved from one-cell ancestors as
simple as modern protozoa beyond the obvious facts of human transformation
during development. They at least are real and not the logical deductions
of reason; yet their very reality and familiarity render us blind to the
deeper meaning revealed to us only when science places the facts in
intelligible order.

* * * * *

And now, in the third p