438 MOEPHOLOGY. Chap. XIII. 



natural selection, during a long-continued course of modi- 

 fication, should have seized on a certain number of the 

 primordially similar elements, many times repeated, and 

 have adapted them to the most diverse purposes. And 

 as the whole amount of modification will have been 

 effected by slight successive steps, we need not wonder 

 at discovering in such parts or organs, a certain degree 

 of fundamental resemblance, retained by the strong 

 principle of inheritance. 



In the great class of molluscs, though we can homo- 

 logise the parts of one species with those of another and 

 distinct species, we can indicate but few serial homo- 

 logies ; that is, we are seldom enabled to say that one 

 part or organ is homologous with another in the same 

 individual. And we can understand this fact ; for in 

 molluscs, even in the lowest members of the class, we 

 do not find nearly so much indefinite repetition of any 

 one part, as we find in the other great classes of the ani- 

 mal and vegetable kingdoms. 



Naturalists frequently speak of the skull as formed of 

 metamorphosed vertebrae : the jaws of crabs as meta- 

 morphosed legs ; the stamens and pistils of flowers as 

 metamorphosed leaves ; but it would in these cases pro- 

 bably be more correct, as Professor Huxley has remarked, 

 to speak of both skull and vertebras, both jaws and legs, 

 &c, — as having been metamorphosed, not one from the 

 other, but from some common element. 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 — verte- 

 brae in the one case and legs in the other — have actually 

 been modified into skulls or jaws. Yet so strong is the 

 appearance of a modification of this nature having oc- 

 curred, that naturalists can hardly avoid employing 

 language having this plain signification. On my view 



