134 ESSAYS OF A BIOLOGIST 



present, however, it appears to have a definite func- 

 tion by making possible, through sexual reproduc- 

 tion, all the various combinations of any heritable 

 variations that may arise in different individuals of 

 a species, and so conferring greater evolutionary plas- 

 ticity on the species as a whole. ^ 



Primarily, sex implies only the fusion of nuclei 

 from two separate individuals; there is no need for 

 sex dififerences to exist at all. Sex differences, how- 

 ever, are almost universal in sexually-reproducing 

 organisms, and represent a division of labour be- 

 tween the active male cell and the passive female 

 cell, the former taking over the task of uniting the 

 two, the latter storing up nutriment for the new indi- 

 vidual that will result from that union. 



The subsequent history of sex is, roughly speak- 

 ing, the history of its invasion of more and more of 

 the organization of its possessors. First the male as 

 a whole, and not merely its reproductive cells, tends 

 to become organized for finding the female. The 

 female's whole type of metabolism is altered to pro- 

 duce the most efficient storage of reserve material in 

 her ova, and later she almost invariably protects and 

 nourishes the young during the first part of their de- 

 velopment, either within or without her own body. 

 Appropriate instincts are of course developed in both 

 male and female. 



At the outset there is enormous waste incurred in 

 the liberation of sperms and ova into the water, there 



2 See East and Jones, '19. 



