T. M. SONNEBORN 205 



cannot be guaranteed to occur; and second choice to genetically 

 less satisfactory, but physiologically equally satisfactory sexual 

 processes, which can be completely depended upon to occur. 

 The second choice is essential to short-term survival; the first 

 choice may be essential to long-term survival. The universality of 

 the occurrence of a first and a second response in time indicates 

 that this is not a mere laboratory phenomenon, but a double 

 assurance mechanism which operates in nature and has been de- 

 veloped and preserved by natural selection. Incidentally, if this 

 is so, the second choice response must occur often enough in 

 nature for selection to be able to preserve it. 



However, the existence of these two sorts of responses in all 

 varieties of P. aurelia does not mean that they are equally well 

 developed in all. As will appear, most of the varietal differences 

 we have set forth in an earlier section bear importantly on just 

 this point. They have the effect of giving greater or lesser em- 

 phasis to one or the other of two successive sexual responses to 

 famine. The life processes of some varieties favor and prolong 

 the possibility of cross conjugation, making them outbreeders. 

 The life processes of other varieties contract these possibilities 

 almost to the vanishing point, making these varieties inbreeders. 

 And other varieties show intermediate conditions. Since out- 

 breeding may be considered advantageous for long-time survival 

 of diploid free-living organisms, it may be assumed that this 

 was the breeding system of the ancestors of the P. aurelia now 

 living. Further, those living varieties that now maintain the out- 

 breeding habit are most likely to provide the solid core of per- 

 sistence on the evolutionary time scale. Relatively few varieties 

 of P. aurelia conform to this exacting requirement; but they are 

 the ones with the widest distributions in nature. On the other 

 hand, inbreeding suffices well enough for short-time survival 

 while conditions remain relatively constant. This mode of life 

 is less exacting. The life cycles succeed one another rapidly so 

 that the system approaches the one in haploids. Evolutionary 

 divergence can and does take place at a rapid rate; but at the 

 sacrifice of some of the genetic plasticity needed for rapid adap- 

 tation. These are the views I shall now attempt to develop in 



