MUTUAL AFFINITIES OF ORGANIC BEINGS 393 



Why should similar bones have been created to form the wing and 

 the leg of a bat, used as they are for such totally different pur- 

 poses, namely, flying and walking? Why should one crustacean, 

 which has an extremely complex mouth formed of many parts, 

 consequently always have fewer legs; or conversely, those with 

 many legs have simpler mouths? Why should the sepals, petals, 

 stamens, and pistils, in each flower, though fitted for such dis- 

 tinct purposes, be all constructed on the same pattern? 



On the theory of natural selection, we can, to a certain extent, 

 answer these questions. We need not here consider how the bodies 

 of some animals first became divided into a series of segments, or 

 how they became divided into right and left sides, with corre- 

 sponding organs, for such questions are almost beyond investiga- 

 tion. It is, however, probable that some serial structures are the 

 result of cells multiplying by division, entailing the multiplication 

 of the parts developed from such cells. It must suffice for our pur- 

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

 or organ is the common characteristic, as Owen has remarked, of 

 all low or httle specialized forms; therefore the unknown pro- 

 genitor 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 

 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 

 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 conditions. Such parts, whether more or less modi- 

 fied, 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 homolo- 

 gies, such as the valves of Chitons, can be indicated; that is, we 

 are seldom enabled to say that one part is homologous with an- 

 other 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 



