238 MORPHOLOGY. [Chap. XIV, 



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. Homo- 

 plastic structures are the same with those which I have 

 classed, though in a very imperfect manner, as analo- 

 gous modifications 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 modifica- 

 tions, having been preserved for the same general pur- 

 pose or function, — of which many instances have been 

 given. 



Naturalists frequently speak of the skull as formed 

 of metamorphosed vertebrae; the jaws of crabs as meta- 

 morphosed legs; the stamens and pistils in flowers as 

 metamorposed leaves; but it would in most cases be 

 more correct, as Professor Huxley has remarked, to speak 

 of both skull and vertebrse, jaws and legs, &e., as hav- 

 ing 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 — vertebra 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 



