commences its existence as an egg,
primarily identical, in all essential respects, with that of the Dog,
but that the yelk of this egg undergoes division--that the primitive
groove arises, and that the contiguous parts of the germ are fashioned,
by precisely similar methods, into a young chick, which, at one stage
of its existence, is so like the nascent Dog, that ordinary inspection
would hardly distinguish the two.

The history of the development of any other vertebrate animal, Lizard,
Snake, Frog, or Fish, tells the same story. There is always, to
begin with, an egg having the same essential structure as that of the
Dog:--the yelk of that egg always undergoes division, or 'segmentation'
as it is often called: the ultimate products of that segmentation
constitute the building materials for the body of the young animal;
and this is built up round a primitive groove, in the floor of which
a notochord is developed. Furthermore, there is a period in which the
young of all these animals resemble one another, not merely in
outward form, but in all essentials of structure, so closely, that the
differences between them are inconsiderable, while, in their subsequent
course, they diverge more and more widely from one another. And it is a
general law, that, the more closely any animals resemble one another
in adult structure, the longer and the more intimately do their embryos
resemble one another: so that, for example, the embryos of a Snake and
of a Lizard remain like one another longer than do those of a Snake and
of a Bird; and the embryo of a Dog and of a Cat remain like one another
for a far longer period than do those of a Dog and a Bird; or of a Dog
and an Opossum; or even than those of a Dog and a Monkey.

Thus the study of development affords a clear test of closeness of
structural affinity, and one turns with impatience to inquire what
results are yielded by the study of the development of Man. Is he
something apart? Does he originate in a totally different way from Dog,
Bird, Frog, and Fish, thus justifying those who assert him to have no
place in nature and no real affinity with the lower world of animal
life? Or does he originate in a similar germ, pass through the same
slow and gradually progressive modifications,--depend on the same
contrivances for protection and nutrition, and finally enter the world
by the help of the same mechanism? The reply is not doubtful for a
moment, and has not been doubtful any time t