GENERATION, SEX AND ONTOGENY 



233 



these backboned animals -- the fishes, the batrachians, the 

 reptiles, the birds, and the mammals have descended from 

 a common ancestor that they all have a backbone. It is 

 believed that the descendants of the first backboned animal 

 have in the course of many generations branched off little by 

 little from the original type until there came to exist very 

 real and obvious differences among the backboned animals- 



E 



FIG. 136. Later stages in the development of the prawn, Peneus potimirium : D, Mysis 



stage; E, adult stage. 



differences which among the living backboned animals are 

 familiar to all of us. The course of development of an in- 

 dividual animal is believed to be a very rapid and evidently 

 much condensed and changed recapitulation of the history 

 which the species or kind of animal to which the developing in- 

 dividual belongs has passed through in the course of its descent 

 through a long series of gradually changing ancestors. If this is 

 true, then we can readily understand why a fish and a salaman- 

 der, a tortoise, a bird and a rabbit, are all much alike, as they 

 really are, in their earlier stages of development, and gradually 

 come to differ more and more as they pass through later and 

 later developmental stages. A crab has a tail in one of its 



