394 THE ORIGIN OF SPECIES 



part as we find in the other great classes of the animal and vege- 

 table kingdoms. 



But morphology is a much more complex subject than it at first 

 appears, as has lately been well shown in a remarkable paper by 

 Mr. E. Ray Lankester, who has drawn an important distinction 

 between certain classes of cases which have all been equally 

 ranked by naturalists as homologous. He proposes to call the 

 structures which resemble each other in distinct animals, owing to 

 their descent from a common progenitor with subsequent modifi- 

 cation, homogenous ; and the resemblances which cannot thus be 

 accounted for, he proposes 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 pro- 

 genitor; 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 successive seg- 

 ments of the same individual animal; and here we have parts com- 

 monly called homologous which bear no relation to the descent of 

 distinct species from a common progenitor. Homoplastic struc- 

 tures are the same with those which I have classed, though in a 

 very imperfect manner, as analogous modifications or resem- 

 blances. Their formation 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 purpose or function, 

 of which many instances have been given. 



Naturalists frequently speak of the skull as formed of meta- 

 morphosed vertebrae; the jaws of crabs as metamorphosed legs; the 

 stamens and pistils in flowers as metamorphosed leaves; but it 

 would in most cases be more correct, as Professor Huxley has re- 

 marked, to speak of both skull and vertebrae, jaws and legs, etc., 

 as having been metamorphosed, not one from the other, as they 

 now exist, but from some common and simpler element. Most 

 naturalists, 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 — vertebrae 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, 



