lid-hoofed pigs mentioned in _Var. under Dom._, Ed. ii.
vol. II. p. 424 are not _Lincolnshire pigs_. For other cases see
Bateson, _Materials for the Study of Variation_, 1894, pp. 387-90.

{459} In the margin C. Bell is given as authority, apparently for
the statement about Plesiosaurus. See _Origin_, Ed. i. p. 436, vi.
p. 598, where the author speaks of the "general pattern" being
obscured in "extinct gigantic sea lizards." In the same place the
suctorial Entomostraca are added as examples of the difficulty of
recognising the type.

{460} _Origin_, Ed. i. p. 438, vi. p. 602.

_Embryology_.

The unity of type in the great classes is shown in another and very
striking manner, namely, in the stages through which the embryo passes
in coming to maturity{461}. Thus, for instance, at one period of the
embryo, the wings of the bat, the hand, hoof or foot of the quadruped,
and the fin of the porpoise do not differ, but consist of a simple
undivided bone. At a still earlier period the embryo of the fish, bird,
reptile and mammal all strikingly resemble each other. Let it not be
supposed this resemblance is only external; for on dissection, the
arteries are found to branch out and run in a peculiar course, wholly
unlike that in the full-grown mammal and bird, but much less unlike that
in the full-grown fish, for they run as if to aerate blood by
branchiae{462} on the neck, of which even the slit-like orifices can be
discerned. How wonderful it is that this structure should be present in
the embryos of animals about to be developed into such different forms,
and of which two great classes respire only in the air. Moreover, as the
embryo of the mammal is matured in the parent's body, and that of the
bird in an egg in the air, and that of the fish in an egg in the water,
we cannot believe that this course of the arteries is related to any
external conditions. In all shell-fish (Gasteropods) the embryo passes
through a state analogous to that of the Pteropodous Mollusca: amongst
insects again, even the most different ones, as the moth, fly and
beetle, the crawling larvae are all closely analogous: amongst the
Radiata, the jelly-fish in its embryonic state resembles a polype, and
in a still earlier state an infusorial animalcule--as does likewise the
embryo of the polype. From the part of the embryo of a mammal, at one
period, resembling a fish more than its parent form; from the larva