It is a fact, long familiar to the world, that the polyp may be cut into without injury, each part soon becoming a complete animal.
To such simplicity of structure the completest contrast is afforded by the higher animals. Throughout their bodies we find a "division of labor," each function having its organ and each organ its distinct function. To trace how this differentiation is carried out would be wearisome, and, being admitted, is fortunately needless.
It may be useful, however, to call to mind the fact that animals which, when mature, are broadly and easily distinguished from each other, are more and more alike the earlier the stage of growth at which we institute a comparison. The differences between a baby chimpanzee and a human infant are much slighter than those between the adults of the respective species. If we extend our researches to the embryonic state we find that the rudimentary man can scarcely be distinguished from many of the other vertebrates. It is only, as Prof. Huxley points out, in the later stages of prenatal growth that the human foetus differs from that of an ape. In the former the convolutions of the brain, according to Prof. Bischoff, reach about the same stage of development as in an adult baboon. The great toe, in man, is considered by Prof. Owen the most characteristic feature of the human skeleton; but, in an embryo about an inch in length. Prof. Wyman found this member not lying parallel with the other toes, but projecting out from the side of the foot as it does permanently in the so-called Quadrumana in their mature condition. Thus plainly does it appear that differentiation is the way to perfection, each animal as it approaches maturity diverging more and more from other forms, from which, in its earlier stages, it was scarcely distinguishable.
Yet again, we may turn from a survey of the growth of the individual, and from a comparison of the highest and lowest forms of contemporary organic life, to the consideration of the successive phases of being that have peopled our earth. Here, too, we find the same great law prevail. In the remote past we find what are called "generalized forms"—animals which seem to have combined in themselves the rough outlines of what we now find developed into perfectly distinct beings.
Suppose it were now proposed as an improvement in the structure of man, or of any other mammalian species, that the functions now exercised by two distinct organs—such as, e. g., the eye and the ear, or the nerves of motion and of sensation—should be "lumped" together, committed to one only set of organs; would such a change, if we for the moment* suppose it practicable, be an advance or a retreat? Would it raise or lower the species in the scale of existence? It might seem a convenience if, instead of seeing with our eyes alone, we could also see and hear with our ears; but would either the seeing or the hearing be done as well as now, when each is the sole function of an express organ? On the principles of the old natural history, as well as of the
