58 Ani7nal Life and Intelligence. 



produce from three or four times to hundreds or even 

 thousands of times as many new individuals as are neces- 

 sary in this way to keep the numbers constant. This is 

 the law of increase. It may be thus stated : The number of 

 individuals in every race or species of animals is tending to 

 increase. Practically this is only a tendency. By war, by 

 struggle, by competition, by the preying of animals upon 

 each other, by the stress of external circumstances, the 

 numbers are thinned down, so that, though the births are 

 many, the deaths are many also, and the survivals few. 

 In the case of those species the numbers of which are 

 remaining constant, out of the total number born only two 

 survive to procreate their kind. We may judge, then, of 

 the amount of extermination that goes on among those 

 animals which produce embryos by the thousand or even 

 the hundred thousand. The effects of this enormous death- 

 rate on the progress of the race or species we shall have 

 to consider in the next chapter, when the, question of the 

 differentiation of species is before us. 



There is one form of differentiation, however, which we 

 may glance at before closing this chapter — the differentia- 

 tion of sex. We are not in a position to discuss the ultimate 

 causes of sex-differentiation, but we may here note the 

 proximate causes as they seem to be indicated in certain 

 cases. 



Among honey-bees there are males (drones) fertile 

 females (queens), and imperfect or infertile females (workers). 

 It has now been shown, beyond question, that the eggs 

 from which drones develop are not fertilized. The presence 

 or absence of fertilization in this case determines the sex. 

 During the nuptial flight, a special reservoir, possessed by 

 the queen bee, is stored with sperms in sufficient number 

 to last her egg-laying life. It is in her power either to 

 fertilize the eggs as they are laid or to withhold fertiliza- 

 tion. If the nuptial flight is prevented, and the reservoir 

 is never stored with sperms, she is incapable of laying 

 anything but drone eggs. The ceUs in which drones are 

 developed are somewhat smaller than those for ordinary 



