318 Heredity. 



no a priori absurdity in the hypothesis that the ovum and 

 the male cell fill different offices. While there is no rea- 

 son for believing that the functions of these elements 

 are alike, there are many reasons for believing that this is 

 not the case; for example, the almost universal occur- 

 rence of differences in form, size, and structure; the 

 possibility of parthenogenesis; the differences between 

 reciprocal hybrids; the fact that the offspring of a mule 

 hybrid and a female of a pure species is much more 

 variable than the offspring of a female hybrid by the 

 male of a pure species; and the fact that a part which is 

 in ore developed or is of more functional importance in 

 the male parent than it is in the female parent, is much 

 more apt to vary in the offspring than a part which is 

 more developed or more important in the mother than it 

 is in the father. 



In the absence of all evidence to the contrary I think 

 we may safely conclude from this positive evidence that a 

 division of physiological labor has arisen during the evo- 

 lution of life, and that the functions of the reproductive 

 elements have became specialized in divergent directions. 



The only way to discover the exact nature of this 

 specialization is to study the influence of each element 

 separately, and the comparison of sexual with asex- 

 ual reproduction is the best available method of doing 

 this, since asexual reproduction is essentially reproduc- 

 tion without a male element, while sexual reproduction 

 is reproduction with a male element. 



Organisms produced from fertilized ova differ from 

 those produced asexually only in their greater tendency 

 to vary, and the hypothesis that the male element has 

 become specialized for the transmission of a tendency to 

 vary naturally suggests itself. Variation is not depen- 

 dent upon fertilization, for plants produced from buds 



