Chap. XIV.] MORPHOLOGY. 237 



their subsequent modification through natural selection, 

 would tend from the first to be similar ; .the^^arts 

 being at an early stage of growth alike, and being 

 subjected to nearly the same conditions. Such parts, 

 whether more or less modified, unless their common 

 origin became wholly obscured, would be serially 

 homologous. 



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 enaljled 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 ]'epetition 

 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 

 animals, owing to tliek' descent from a common 

 progenitor with subsequent modification, homogenous ; 

 and the resemblances which cannot thus be accounted 

 for, he proposes to call homojjlastic. 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 



