MORPHOLOGY 393 



the benefit derived from the yielding of the separate 

 pieces in the act of parturition of mammals, will 

 by no means explain the same construction in the 

 skulls of birds. Why should similar bones have been 

 created in the formation of the wing and leg of a bat, 

 used as they are for such totally different purposes ? 

 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 any individual flower, 

 though fitted for such widely different purposes, be all 

 constructed on the same pattern ? 



On the theory of natural selection, we can satisfactorily 

 answer these questions. In the vertebrata, we see a series 

 of internal vertebrae bearing certain processes and appen- 

 dages ; in the articulata, we see the body divided into a 

 series of segments, bearing external appendages ; and in 

 flowering plants, we see a series of successive spiral 

 whorls of leaves. An indefinite repetition of the same 

 part or organ is the common characteristic (as Owen 

 has observed) of ail low or little-modified forms ; there- 

 fore we may readily believe that the unknown progenitor 

 of the vertebrata possessed many vertebrae ; the unknown 

 progenitor of the articulata, many segments ; and the 

 unknown progenitor of flowering plants, many spiral 

 whorls of leaves. We have formerly seen that parts many 

 times repeated are eminently liable to vary in number 

 and structure ; consequently it is quite probable that 

 natural selection, during a long-continued course of 

 modification, should have seized on a certain number of 

 the primordially similar elements, many times repeated, 

 and have adapted them to the most diverse purposes. 

 And as the whole amount of modification will have been 

 effected by slight successive steps, we need not wonder 

 at discovering in such parts or organs, a certain degree 

 of fundamental resemblance, retained by the strong 

 principle of inheritance. 



In the great class of molluscs, though we can homo- 

 logise the parts of one species with those of other and 



