Chap. XIV.] MORPHOLOGY. 237 



would tend from the first to be similar; the parts he- 

 mg at an early stage of growth alike, and heing sub- 

 jected to nearly the same conditions. Such parts, 

 whether more or less modified, unless their common 

 origin became wholly obscured, would be serially homo- 

 logous. 



In the great class of molluscs, though the parts in 

 distinct species can be shown to be homologous, only a 

 few serial homologies, such as the valves of Chitons, 

 can be indicated; that is, we are seldom enabled to say 

 that one part is homologous with another part 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. 



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. Eay 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 ani- 

 mals, owing to their descent from a common progenitor 

 with subsequent modification, homogenous; and the re- 

 semblances which cannot thus be accounted for, he pro- 

 poses to call homoplastic. For instance, he believes that 

 the hearts of birds and mammals are as a whole homo- 

 genous, — 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 inde- 

 pendently developed. Mr. Lankester also adduces the 

 close resemblance of the parts on the right and left sides 



