394 ON THE ORIGIN OF SPECIES 



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 

 animal 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 prob- 

 ably be more correct, as Professor Huxley has remarked, 

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

 etc., — 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- 

 bra? 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 

 occurred, that naturalists can hardly avoid employing 

 language having this plain signification. On my view 

 these terms may be used literally ; and the wonderful 

 fact of the jaws, for instance, of a crab retaining 

 numerous characters, which they would probably have 

 retained through inheritance, if they had really been 

 metamorphosed during a long course of descent 

 from true legs, or from some simple appendage, is 

 explained. 



Embryology. — It has already been casually remarked 

 that certain organs in the individual, which when mature 

 become widely different and serve for different purposes, 

 are in the embryo exactly alike. The embryos, also, of 

 distinct animals within the same class are often strikingly 

 similar : a better proof of this cannot be given, than a 

 circumstance mentioned by Agassiz, namely, that having 

 forgotten to ticket the embryo of some vertebrate animal, 



