246 THE ORIUIN OF iU'EVlES 



segments; and the unknown progenitor of flowering 

 plants, many leaves arranged in one or more spires. We 

 have also formerl}^ seen that parts many times repeated 

 are eminently liable to vary, not only in number, but in 

 form. Consequently such parts, being already present 

 in considerable numbers, and being highly variable, 

 would naturally afford the materials for adaptation to 

 the most different purposes; yet they would generally 

 retain, through the force of inheritance, plain traces of 

 their original or fundamental resemblance. They wouki 

 retain this resemblance all the more, as the variations, 

 which afforded the basis for their subsequent modifica- 

 tion through natural selection, would tend from the first 

 to be similar; the parts 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 mollusks, 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 

 mollusks, 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. Ray Lankester, who has 

 drawn an important distinction between certain classes of 



