362 MORPHOLOGY. [CHAP. XIV. 



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

 vegetable 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. Kay 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 

 modification, homogenous ; and the resemblances which cannot 

 thus be accounted for, he proposes to call liomoplastic. 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 

 successive segments of the same individual animal ; and here we 

 have parts commonly called homologous, which bear no relation 

 to the descent of distinct species from a common progenitor. 

 Homoplastic structures are the same with those which I have 

 classed, though in a very imperfect manner, as analogous modifi- 

 cations or resemblances. 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 metamor- 

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

 stamens and pistils in flowers as metamorphosed leaves ; but 

 would in most cases be more correct, as Professor Huxley 

 remarked, to speak of both skull and vertebrae, jaws and legs, &e 

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

 now exist, but from some common and simpler element. Mo: 

 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, sue 

 language may be used literally; and the wonderful fact of th 

 jaws, for instance, of a crab retaining numerous characters, whici 

 they probably would have retained through inheritance, if th 

 had really been metamorphosed from true though extreme!, 

 simple legs, is in part explained. 





