134 ESSAYS OF A BIOLOGIST 



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

 ever, are almost universal in sexually-reproducing 

 organisms, and represent a division of labour between 

 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 individual 

 that will result from that union. 



The subsequent history of sex is, roughly speaking, 

 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 

 development, 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 

 to unite as best they may. Congress of the sexes 

 eliminates the major part of this waste, and is universal 

 above a certain level. This is in itself the basis for 

 other changes. As the mind, or shall we say the 

 psycho-neural organization, becomes more complex, 

 the sexual instinct becomes more interwoven with 

 the general emotional state ; and a large number 

 of animals appear no^t to mate unless their emotional 



