with gills. The embryos of
wildcats and dogs, rabbits and rats, pigs, deer, and sheep, and of all
other mammalia, possess similar structures. Thus they all pass through a
stage which is found also in the development of reptiles, birds, and
amphibia,--a stage which corresponds to the fish throughout its life.
Unless these facts mean that the great classes of vertebrates have
originated together from the same or closely similar ancestors, they are
unintelligible; for we cannot see why a cat or a chick should have to be
essentially fishlike at any time unless this is so. Comparative anatomy
states as we have learned that the amphibia as a class have evolved from
and have out-developed the fishes, that reptiles have progressed still
higher, and that birds and mammals have originated from reptilian
ancestors along roads that have diverged beyond the immediate parent
class. Because the members of each class have to pass along the same path
trodden by their many varied ancestors, although at express speed, as it
were, the similarity of the earliest stages in their development is
explained, for during these periods they are traversing a path over which
their ancestors passed together.

The places where the developing embryos depart from the common mode show
where the several divisions took leave of one another in their
evolution,--a point that comes out with great clearness when the facts of
mammalian development are broadly compared. The embryos of carnivora and
rodents and hoofed animals are alike in their earlier development, and
their agreement means a community of origin. At a certain point the cat and
dog depart from the common mode, but they remain alike up to a far later
stage than the one in which they are similar to the embryos of rats and
sheep. The rat and squirrel and rabbit, on their part, remain together
until long after they take leave of the carnivora and ungulates; while the
sheep and cattle and pigs have their own branch line, which they follow in
company after leaving the embryos of the other orders. The reasons for
these facts seem to be that the members of the three orders exemplified
have evolved from the same stock, which accounts for their embryonic
similarity for a long time after they collectively come to differ from
amphibia and reptiles, while the members in each order became
differentiated only later, wherefore their embryonic paths coincide for a
longer period. Thus the degree of adult resemblance