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. 



