CiiAr. XIII. MOKPIIOLOGY. 393 



posed of such nimicrous and such cxtraordinarily-shajicd pieces 

 of bone ? As Owen has remarked, the benefit derived from 

 tlie yiokling of the separate pieces in the act of j^arturition of 

 nianiiiials, will by no means explain the same construction in 

 the sl-:ulls of birds and reptiles. Why should similar bones 

 have been created to form the wiufi;' and the leg- of a bat, used 

 as they are for such totally diflcrcnt purposes? Why should 

 one crustacean, which has an extremely complex mouth formed 

 of many parts, conseciuently always have fewer leg's ; or, con- 

 versely, those with many legs have sim])ler mouths? ^Vhy 

 should the sepals, petals, stamens, and pistils, in each flower, 

 tlunigh litted for such widely-different purposes, be all con- 

 structed on the same pattern ? 



On the theory of natural selection, we can answer these 

 ({uestions. In the vertebrata, Ave see a series of internal verte- 

 bra? bearing certain processes; in the arliculata, the body di- 

 vided into a series of segments, bearing' external appendag^es ; 

 and, in flowering' plants, spiral whorls of leaves. An indefinite 

 repetition of the same part or organ is the common character- 

 istic, as Owen has observed, of all low or little-modified forms; 

 therefore the unknown progenitor of the vertebrata no doubt 

 possessed man}' vertebrae ; the unknown progenitor of the ar- 

 liculata, many segments; and the unknown progenitor of 

 (lowering ])lants, many leaves arranged in one or more spires. 

 AV(5 have formerly seen that parts many times repeated are 

 eminently liable to vary in number and structure. Conse- 

 (juenlly, such ])arts being already present, and being highly 

 variable, would atford the materials for adaptation to the most 

 different purposes; and they would generally retain through 

 the force of inheritance plain traces of their original or funda- 

 mental resemblance. 



In the great class of moUusks, though it can easily be 

 shown that the parts in distinct species are homologous, but 

 few serial homologies can be indicated ; that is, we are seldom 

 enabled to say that one part is homologous with another jiart 

 in the same individual. And we can imdersfand this fact ; for 

 ill 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 

 ]>:ingdoms. 



Naturalists frequently speak of the skull as fonnc.d of meta- 

 morphosed verlebni' ; the jaws of crabs as metamorphosed legs; 

 the stamens and pistils in flowers as metamorphosed leaves ; but 



