Embryology, the study of early development in the individual animal or plant, also throws much side light upon the nature and ancestry of each species or family. For example, gorse, which is a member of the pea-flower tribe, has in its adult stage solid, spiny, thorn-like leaves, none of which in the least resemble the foliage of the clover, to which it is closely related; but the young seedling in its earliest stages has trefoil leaves, which only slowly pass by infinitesimal gradations into flat blades and finally into the familiar defensive prickles. Here, natural selection under stress of herbivorous animals on open heaths and commons has spared only those particular gorse-bushes which varied in the direction of the stiffest and most inedible foliage; but the young plant in its first days still preserves for us the trefoil leaf which it shared originally with a vast group of clover-like congeners. The adult barnacle, once more, presents a certain fallacious external resemblance to a mollusk, and was actually so classed even by the penetrating and systematic intellect of Cuvier; but a glance at the larva shows an instructed eye at once that it is really a shell-making and abnormal crustacean. On a wider scale, the embryos of mammals are at first indistinguishable from those of birds or reptiles; the feet of lizards, the hoofs of horses, the hands of man, the wings of the bat, the pinions of birds, all arise from the same fundamental shapeless bud, in the same spot of an almost identical embryo. Even the human foetus, at a certain stage of its development, is provided with gill-slits, which point dimly back to the remote ages when its ancestor was something very like a fish. The embryo is a picture, more or less obscured and blurred in its outline, of the common progenitor of a whole great class of plants or animals.
Finally, classification points in the same way to the affiliation of all existing genera and species upon certain early divergent ancestors. The whole scheme of the biological system, as initiated by Linnæus and improved by Cuvier, Jussieu, De Candolle, and their successors, is essentially that of a genealogical tree. The prime central vertebrate ancestor--to take the case of the creatures most familiar to the general reader--appears to have been an animal not unlike the existing lancelet, a mud-haunting, cartilaginous, undeveloped fish, whose main lineaments are also embryologically preserved for us in the ascidian larva and the common tadpole. From this early common centre have been developed, apparently, in one direction the fishes, and in another the amphibian tribes of frogs, newts, salamanders, and axolotls. From an early amphibian, again, the common ancestor of birds, reptiles, and mammals seems to have diverged: the intermediate links between bird and reptile being faintly traced among the extinct deinosaurians and the archæopteryx, some years subsequently to the first appearance of the 'Origin of Species;' while the ornithorhynchus, which to some extent connects the mammals, and especially the marsupials, with the lower egg-laying types of vertebrate, was already well-known and thoroughly studied before the publication of Darwin's great work. Throughout, the indications given by all the chief tribes of animals and plants point back to slow descent and divergence from common ancestors; and all the subsequent course of palæontological research has supplied us rapidly, one after another, with the remains of just such undifferentiated family starting-points.