T. M. SONNEBORN 221 



conditions of the longest immature and mature periods, the 

 chance of outbreeding may still be small. But the occurrence of 

 selfing during early senility in varieties with such long periods 

 starts a new cycle and extends the time available for finding 

 suitable mates. 



Second, if the total life prior to senility occupies a year or more, 

 the annual rise and fall in population density may reduce the 

 chance of inbreeding by rendering the population less hetero- 

 geneous in mating type. 



Third, the immature and mature periods must be considered 

 together sometimes to avoid making an erroneous judgment based 

 on one of them alone. According to old observations of mine on 

 stock S of variety 1, it has an immature period that lasts almost 

 until the beginning of senility and that leaves but a very brief ma- 

 ture period. The immature period alone suggests an outbreeder, 

 but the mature period is so brief as to militate strongly against 

 mating with members of another population and even against 

 mating at all. Unless famine comes at just the right moment, its 

 consequence would be autogamy or no sexual process at all. How- 

 ever, since no immature period follows autogamy and the mature 

 period is then correspondingly longer, there is more opportunity 

 for the occurrence of conjugation after an autogamy in such a 

 strain. This example shows how the interplay of various aspects 

 of life must be taken into account before arriving at a judgment 

 of the breeding system. 



In addition to the duration of the periods of immaturity and 

 maturity, there are two other important signs of the breeding sys- 

 tem. One is a direct examination of the frequencies of various 

 genotypes in a natural population. An excess of homozygotes and 

 a deficiency of heterozygotes, in comparison with expectations 

 based on random mating, is of course a sign of inbreeding even 

 within a local population. The other sign is the karyological pic- 

 ture or inferences about it based upon the nature of the F2 after 

 crosses between representatives of different populations. Kar- 

 yological differences between local populations and increased F2 

 mortality following crosses between them indicate that the popu- 

 lations are isolates of inbreeders. The indication is confirmed if 



