120 Life and Death, Heredity and Evolution 



each individual of the pair likewise produces a larger inac- 

 tive half nucleus, which remains passive and is sought out 

 by the smaller one; this inactive nucleus plays the role 

 of the egg in ordinary fertilization. If there is a funda- 

 mental sex difference between the two uniting half nuclei, we 

 must call the active one male, the passive one female. But 

 the two individuals that unite both produce male and female 

 nuclei; each would have to be characterized either as both 

 male and female, or as neutral. There appears a difficulty 

 here, for why, if the two individuals are alike, should they 

 be drawn together and mate? According to the theory it is 

 unlikeness of sex that brings about mating. 



Attempts have been made to show that the two individuals 

 which conjugate really are of preponderatingly different 

 sexes ; that one exceeds the other in maleness, the other in 

 femaleness, if we may so speak; this would avoid the diffi- 

 culty we have mentioned. Calkins (1902) pointed out that 

 often only one of the two individuals that have mated repro- 

 duces freely, the other reproducing but weakly or dying with- 

 out reproduction; Miss Cull (1907) showed that this hap- 

 pens in a great number of cases. The individual that pro- 

 duces young after mating would be mainly female, while the 

 other would play the role of male. Of course the differentia- 

 tion of sexes was not held to have gone far in such a case ; 

 all that is needed is that one should be, as it were, more 

 female than the other; then the diversity would operate to 

 produce mating and consequent rejuvenescence. 



This idea is rendered plausible by the fact that in some 

 other infusoria, such as Vorticella and its relatives, there is 

 an observable difference between the two individuals that 

 conjugate. One of the two mates is small and active, taking 

 the usual role of the male, while the other is larger and 

 remains quiet, taking the part of the female (Figures 33 



