T. M. SONNEBORN 299 



serves to give outbreeders an opportunity to migrate away from 

 close relatives before they can mate; short or no immaturity per- 

 mits inbreeders to mate when they are still nearest to their close 

 relatives. Long maturity gives outbreeders time to find suitable 

 mates; short maturity in inbreeders restricts the opportunity to 

 mate with others. Long senility, during which mating is still 

 possible but less likely to yield viable progeny, occurs in out- 

 breeders and extends the opportunity to find suitable mates; the 

 relatively short senility of inbreeders is characterized by inability 

 to mate at all, or by greatly reduced ability to mate. During 

 senility in inbreeders, the form taken by fertilization is autogamy, 

 the closest form of inbreeding; and this stage of life is by far 

 the longest, immaturity being contracted virtually to nothingness 

 and maturity lasting only a matter of some days. Outbreeders do 

 not undergo autogamy. If they fail to find a suitable unrelated 

 mate, eventually complementary mating types arise within a 

 single line of descent and conjugation occurs within the line. 

 This serves to rejuvenate, initiating a new life cycle, and at the 

 same time serves to maintain heterozygosity, which is not accom- 

 plished by autogamy. 



Outbreeders commonly have systems of multiple interbreeding 

 mating types, while inbreeders commonly have but two. Multiple 

 types serve to increase the possibility of mating when an out- 

 breeder meets a stranger. The method of mating type determina- 

 tion in outbreeders results in uniformity of mating type of the 

 synclone (the two clones from a pair of conjugants). This pre- 

 vents the closest forms of inbreeding. Inbreeders, however, regu- 

 larly yield more than one mating type in a synclone; this serves 

 to make the closest relatives capable of mating with each other. 

 The several systems of mating type inheritance (except the one 

 in Euplotes ) are based on a single fundamental system of mating 

 type determination, with relatively minor differences in detail. 



In asexual Protozoa, the most important differences in life 

 features are with respect to the rapidity of reproduction, the size 

 of populations, the ploidy level, and the constancy or variability 

 of the conditions of life. Haploids that reproduce rapidly into 

 large populations can probably depend upon mutations to provide 



