78 Heredity. 



which have produced the various forms of animal and 

 vegetable life, will guide us nearer to the truth than the 

 speculations of the last century. Bonnet and Haller 

 might fairly assume that each species had been what it 

 is now "from the beginning," but we cannot nowaday 

 make any such assumption, and we must believe that 

 the structure of the germ, like the structure of the 

 adult animal, has been gradually acquired by natural 

 selection. 



A modern hypothesis of evolution must therefore be a 

 very different thing from the one which Bonnet fur- 

 nished, and must account for the slow advancement of 

 the germ from generation to generation. 



In Darwin's pangenesis hypothesis we have a provi- 

 sional explanation based upon the generalizations of 

 modern science. It is a true evolution hypothesis, for 

 Darwin believes that an ovum or a male cell is a wonder- 

 fully complex structure, and that it contains gemmules 

 to represent each feature in the organization of the 

 adult. One essential difference between this hypothesis 

 and the original hypothesis of evolution Us stated by 

 Bonnet, is that Darwin believes that the ovum contains, 

 not the perfect animal in miniature, but a distinct germ 

 for each distinct cell or structural element of the adult. 

 Darwin's hypothesis recognizes the gradual specializa- 

 tion of the ovum during the evolution of the race, for 

 each cell of the body of the parent may at any time 

 transmit to it new gemmules. Most of the objections to 

 it are based upon its complexity, and on the almost in- 

 finite number of gemmules which it requires; but besides 

 these objections we know from G-alton's experiments 

 that it is impossible to accept it without modification. 

 We also have, in the fact that the functions of the two 

 sexual elements are not alike, a reason for believing that, 



