T. M. SONNEBORN 203 



reproduction so that they become widely distributed prior to 

 recombination. 



In Paramecium, and in Ciliates generally, the occurrence of 

 sexual processes and genetic recombination is linked to decline 

 in the food supply. This must be an inevitable and frequently re- 

 curring event in the lives of such organisms. The logarithmic in- 

 crease in numbers resulting from repeated binary fission quickly 

 yields populations that no conceivable food supply could support. 

 Not even an amount of food equal to the bulk of the earth could 

 support reproduction from a single individual for as much as 

 100 successive fissions. With due allowance for predation and 

 other causes of accidental death, it is difficult to see how asexual 

 reproduction could long continue without the onset of famine. 

 The prevailing pattern of life must be to increase, multiply, and 

 spread while the harvest lasts, forgetting about sex while enjoying 

 food to the full, and to use hard times when reproduction comes 

 to a virtual standstill as occasions for grasping opportunities to 

 carry out sexual processes. 



Little is yet known about the alternation of asexual reproduc- 

 tion and sexual processes in nature. We are at present largely 

 limited to inferences based on laboratory study. What is worse, 

 we systematically carry out the laboratory studies under con- 

 ditions that differ in important respects from those which prob- 

 ably exist in nature. For example, we routinely and at once trans- 

 fer fertilized animals from the starvation conditions that induced 

 fertilization to conditions of excess in available food. In nature, 

 the conditions that induce fertilization would hardly be expected 

 to change so radically in the course of the hours occupied by the 

 processes of meiosis and fertilization. Yet we know very little 

 about the effects of this difference between natural and labora- 

 tory conditions. That it does have some effects on the nuclear 

 processes following fertilization and even on the subsequent 

 breeding relations is already indicated. The whole matter is in 

 great need of investigation. 



Nevertheless the laboratory studies seem clearly to show that 

 there are two distinct and independent functions of sexual proc- 

 esses. First, they have the usual function of genetic recombina- 



