298 PROTOZOA 



progressively more limited with the passage through various 

 grades of breeding systems from extreme outbreeding to extreme 

 inbreeding. As the extreme of inbreeding is approached, each of 

 the many local populations of a Ciliate actually approaches in its 

 isolation and genetic differentiation the status of a syngen. Al- 

 though evolutionary divergence has not yet passed the point of 

 no potential for return, the actual level of biological organization 

 is as different as possible from that observed in outbreeding 

 syngens. Yet all gradations are found from one extreme to the 

 other. Although little information is available on asexual Protozoa, 

 the degree of isolation and genetic differentiation of local popu- 

 lations supposed to exist in them could be at most but little more 

 than actually occurs near the inbreeding extreme of the series 

 in sexual organisms. Both the potential and the actual group of 

 interbreeding populations contracts progressively with progressive 

 changes in the breeding system and the final change in method 

 of reproduction. The significance of the syngen as an objective 

 and real level of biological organization correspondingly de- 

 creases as the local population progressively increases in such 

 significance. 



The Significance of Differences in the Major Features of Life. 

 Among the Ciliates, differences in the major features of life ap- 

 pear chiefly as quantitative variations in the duration of the 

 various stages of a fundamentally similar life cycle, as variations 

 in the details of a fundamentally similar mechanism of mating 

 type determination, and as variations in the form of fertilization 

 characteristic of the senile stage of life. Previously these varia- 

 tions were known as mere brute facts. In this paper they have 

 been shown to be of the greatest significance as adaptations to 

 the breeding system by which the syngen perpetuates itself. 



Variations in the duration of stages in the life cycle are found 

 with respect to periods of immaturity, maturity, senility, and 

 total life. These are all long in outbreeders, short in inbreeders, 

 and intermediate in those with intermediate breeding systems. 

 The same sort of general relation has been pointed out in certain 

 plants by Stebbins (1950): self-fertilizers are chiefly annuals and 

 outbreeders are chiefly perennials. In Ciliates, long immaturity 



