THE EVOLUTION OF SEX 11 



numbers of individuals, the maximum division rate is 

 kept up, because, at any one time, the majority of the 

 individuals come from the combinations favorable to 

 that special environment. 



There are certain points in this argument that call 

 for further consideration. In a mass culture the fa- 

 vorable combinations for that culture will soon be made, 

 if conjugation is taking place. At least this is true if 

 such combinations are homogeneous (homozygous, in 

 technical language). Under such circumstances the 

 race will become a pure strain, and further conjugation 

 could do nothing for it even if it were transferred to a 

 medium unsuited to it. 



In the ordinary division of a cell every single de- 

 terminer divides and each of the new cells receives 

 half of each determiner. Hence in the case of para- 

 mcecium all the descendants of a given paramcecium 

 that are produced by division must be exactly alike. 

 But in preparation for conjugation a different pro- 

 cess may be supposed to take place, as in higher 

 animals, among the determiners. The determiners 

 unite in pairs and then, by division, separate from 

 each other, Fig. 4. In consequence the number of 

 determiners is reduced to half. Each group of deter- 

 miners will be different from the parent group, pro- 

 vided the two determiners that united were not 

 identical. If after this has occurred conjugation 

 takes place, the process not only restores the total 

 number of determiners in each conjugant, but gives 

 new groups that differ from both of the original 

 groups. 



The maintenance of the equilibrium between an 



