. XIV. | irfORPHOLOGY. 361 



Owen has remarked, the benefit derived from the yielding of the 

 separate pieces in the act of parturition by mammals, will by no 

 means explain the same construction in the skulls of birds and 

 reptiles. 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 purposes, 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 distinct 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 corresponding organs, for such questions are almost beyond 

 investigation. 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 purpose 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 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 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 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 homo- 

 logies, 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 



