384 



THE EVOLUTION OF LIFE. 



that thooe numeroiis lateral appendages which, in the lowex 

 crustaceans most of them serve as legs, and have like shapos> 

 are, in the higher crustaceans, some of them represented by 

 enormousljr-developed claws, and others by variously-modi- 

 fied foot-jaws. " It is familiar to almost every one," he 

 continues, "that in a flower the relative position of tho 

 sepals, petals, stamens, and pistils, as well as their iatimate 

 structure, are intelligible on the view that they consist of 

 metamorphosed leaves arranged iu a spire. In monstrous 

 plants we often get direct evidence of the possibility of one 

 organ being transformed into another ; and we can actually 

 see in embryonic crustaceans and in many other animals, and 

 in flowers, that organs, which when mature become extremely 

 difierent, are at an early stage of growth exactly alike." 

 * * * " Why should one crustacean, which has an ex- 

 tremely 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 pistHs ia any individual flower, though fitted 

 for such widely-different purposes, be all constructed on the 

 same pattern ? " 



To these and countless similar questions, the theory of 

 evolution furnishes the only rational answer. In the course 

 of that change from homogeneity to heterogeneity of struc- 

 ture, displayed in evolution under every form, it will neces- 

 sarily happen that from organisms made up of numerous 

 like parts, there will arise organisms made up of parts more 

 and more unlike : which unlike parts will nevertheless con- 

 tinue to bear traces of their primitive likeness. 



§ 185. One more striking morphological fact, near akin 

 to some of the facts dwelt on in the last chapter, must be 

 here set down — the frequent occurrence, in adult animals 

 and plants, of rudimentary and useless organs, which are 

 homologous with organs that are developed and useful in 

 allied animals and plants. In the last chapter we saw that 



