Page - 477 - in The Origin of Species

MORPHOLOGY 477 resemblances which cannot thus be accounted for, he pro- poses to call homoplastic. For instance, he believes that the hearts of birds and mammals are as a whole homogenous,— that is, have been derived from a common progenitor ; but that the four cavities of the heart in the two classes are homoplastic,—that is, have been independently developed. Mr. Lankester also adduces the close resemblance of the parts on the right and left sides of the body, and in the suc- cessive segments of the same individual animal ; and here we have parts commonly called homologous, which bear no rela- tion to the descent of distinct species from a common pro- genitor. Homoplastic structures are the same with those which I have classed, though in a very imperfect manner, as analogous modifications or resemblances. Their forma- tion may be attributed in part to distinct organisms, or to distinct parts of the same organism, having varied in an analogous manner; and in part to similar modifications, having been preserved for the same general pui ^se or func- tion,—of which many instances have been given. Naturalists frequently speak of the skull as formed of metamorphosed vertebrae ; the jaws of crabs as metamor- phosed legs; the stamens and pistils in flowers as meta- morphosed leaves ; but it would in most cases be more correct, as Professor Huxley has remarked, to speak of both skull and vertebrce, jaws and legs, &:c., as having been metamorphosed, not one from the other, as they now exist, but from some common and simpler element. Most natu- ralists, however, use such language only in a metaphorical sense ; they are far from meaning that during a long course of descent, primordial organs of any kind—vertebrai in the one case and legs in the other—have actually been converted into skulls or jaws. Yet so strong is the appearance of this having occurred, that naturalists can hardly avoid employing language having this plain signification. According to the views here maintained, such language may be used literally; and the wonderful fact of the jaws, for instance, of a crab retaining numerous characters, which they probably would have retained through inheritance, if they had really been metamorphosed from true though extremely simple legs, is in part explained.