He speaks likewise of the unity of plan. Every creature is built differently, and the difference is especially striking between members of the several grand divisions of the animal kingdom. The plan then of each is different, and so is the typical idea which prescribes its formation. No animal can then be considered as the proof or outline of another. "Is there a unity of plan? The plan is the relative location of the parts. One can conceive very well the unity of plan without the unity of number; for it is sufficient that all the parts, whatever their number may be, keep always relatively to each other the same place. But can one say that the vertebrate animal, whose nervous system is placed above the digestive canal, is fashioned after the same plan as the mollusk, whose digestive canal is placed above the nervous system? Can one say that the crustacean, whose heart is placed above the spinal marrow, is fashioned after the same pattern as the vertebrate, whose spinal marrow is placed above the heart? Is the relative location of the parts maintained? On the contrary, is it not overthrown? And if there is a change in the location of parts, how is there a unity of plan?" Müller draws nearer to the consideration of the development of the human embryo, and forcibly illustrates the falsehood of the pretended theory. "It is not long since it was held with great seriousness that the human foetus, before reaching its perfect state, travels _successively_ though the different degrees of development which are permanent during the whole life of animals of inferior classes. That hypothesis has not the least foundation, as Baer has shown. The human embryo never resembles a radiate, or an insect, or a mollusk, or a worm. The plan of formation of those animals is quite different from that of the vertebrate. Man then might at most resemble these last, since he himself is a vertebrate, and his organization is fashioned after the common type of this great division of the animal kingdom. But he does not even resemble at one time a fish, at another a reptile, a bird, etc. The analogy is no greater between him and a reptile or a bird, than it is between all vertebrate animals. During the first stages of their formation, all the embryos of vertebrate animals present merely the simplest and most general delineations of the type of a vertebrate; hence it is that they resemble each other so much as to render it very difficult to distinguish them. The fish, the reptile, the bird, the mammal, and man are at first the simplest expression of a type common to all; but in proportion as they grow, the general resemblance becomes fainter and fainter, and their extremities, for instance, after being alike for a certain time, assume the characters of wings, of hands, of feet, etc." Mr. Milne-Edwards takes the same view of embryonic generation: