258 THE SOCIAL LIFE OF ANIMALS 



males, so that overcrowding may be rated as a time 

 of environmental stress. Either by the shortage of 

 food, by the accumulation of waste products, or 

 from some other cause, the association of many fe- 

 male cladocerans together results in the production 

 of eggs which have a different prospective potency 

 from those the same females, uncrowded, would 

 produce; and sexual males and females are the result. 



It is evident from these varying examples that 

 even the fundamental matter of sex, with the caste* 

 like divisions of labor that result from two sexes, 

 may be determined by the close association of ani- 

 mals of the same species. There is some reason, 

 though perhaps it is slight, for suggesting as in 

 Chapter III that sex itself may have grown origi- 

 nally out of mutual acceleration in division rates 

 when two or more primitive organisms were in close 

 contact in small space. The whole matter of sex may 

 hark back to some of the basic aspects of mass physi- 

 ology which were set forth earlier in this book. 



Sex in its different aspects plays a highly impor- 

 tant role in the social affairs of animals. It is inter- 

 esting to find that this fundamental cleavage through 

 so much of animal life can at times be controlled 

 by group relationships. Such considerations serve 

 again to emphasize the difficulty of drawing a hard 



