476 ORIGIN OF SPECIES 



developed from such cells. It must suffice for our purpose 

 to bear in mind that an indefinite repetition of the same part 

 or organ is the common characteristic, as Owen has re- 

 marked, of all low or little specialised forms; therefore the 

 unknown progenitor of the Vertebrata probably possessed 

 many vertebrae ; the unknown progenitor of the Articulata, 

 many segments; and the unknown progenitor of flowering 

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

 have also formerly 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 consider- 

 able numbers, and being highly variable, would naturally 

 afford the materials for adaptation to the most different pur- 

 poses ; yet they would generally retain, through the force 

 of inheritance, plain traces of their original or fundamental 

 resemblance. They would retain this resemblance all the 

 more, as the variations, which afforded the basis for their 

 subsequent modification 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 con- 

 ditions. Such parts, whether more or less modified, unless 

 their common origin became wholly obscure, would be se- 

 rially 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 enabled to say that one part is homol- 

 ogous 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 indefi- 

 nite 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 cases which have all 

 been equally ranked by naturalists as homologous. He pro- 

 poses to call the structures which resemble each other in 

 distinct animals, owing to their descent from a common pro- 

 genitor with subsequent modification, homogenous; and the 



