{458} The solid-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 larvae of all orders of insects more resembling the simpler articulate animals than their parent insects{463}; and from such other cases as the embryo of the jelly-fish resembling a polype much nearer than the perfect jelly-fish; it has often been asserted that the higher animal in each class passes through the state of a lower animal; for instance, that the mammal amongst the vertebrata passes through the state of a fish{464}: but Mueller denies this, and affirms that the young mammal is at no time