THE FACTS AND FUNCTION OF SEX 20 



already seen that heredity is inevitable in the one- 

 celled animals and plants; but Weismann's idea of 

 the germ-plasm as continuous and the individual as 

 merely its temporary trustee loads us to look upon 

 heredity as equally inevitable, even in the case of 

 the multicellular organisms. Now where, as in 

 parthenogenesis, there is no complication, the new 

 individual arising from the unassisted ovum of the 

 old, we might reasonably expect heredity to bo very 

 exact and variation either absent altogether or very 

 slight. In accordance with his theory of the func- 

 tion of sex — immediately to be considered — Weis- 

 mann taught that parthenogenetic species do not 

 vary, cannot vary, since it is the intermingling of 

 sexual cells in bi-parental reproduction that, as he 

 averred, gives rise to variations. 



This matter has, therefore, been carefully studied 

 of late years, and it is now possible positively to 

 deny the contention of Weismann. Variation occurs 

 abundantly even in asexvxil and parthenogenetic 

 repi'oduction. This is a fact of cardinal importance, 

 and must be remembered as profoundly aliecting our 

 understanding^ of the cause of variations. Havinsf 

 carefully noted it, we are now prepared to consider 

 the facts of heredity and variation as observed in 

 ordinary sexual or bi-parental reproduction, which is 

 so widely observed throughout the animal and vege- 

 table kinsfdoms. 



In the first place, we must discuss the observed 

 facts of this method of reproduction. We have 

 noted that the gametes, or sex-cells, are of two 

 kinds, male and female. Each of these is a complete 

 cell ; we have seen that the ovum — in certain low 



