PROTOZOA 



each population alone survives inbreeding very well. Outbreeders 

 would be expected to show inbreeding mortality as a result of 

 lethal and detrimental genie recombinations, but this should hap- 

 pen both when the inbreeding follows crosses and when it occurs 

 within a single population. 



Certain other indices of the breeding system are perhaps rela- 

 tively minor in comparison with those already discussed, but they 

 are still significant. First, the range of distribution of a variety is 

 both a rough measure of the number of populations available for 

 interbreeding and an index of the genetic heterogeneity of the 

 variety. Outbreeders generally have a wide range of distribution 

 both in latitude and longitude. Inbreeders are generally restricted 

 either in latitude or in both directions. Second, the rates at which 

 the processes of meiosis, fertilization, postzygotic nuclear reorgan- 

 ization, and particularly asexual reproduction occur are relatively 

 slower in the extreme outbreeders than in the extreme inbreeders. 

 The slower the tempo, the more time for dispersal before matur- 

 ity and before senility, and therefore the more favorable this is for 

 outbreeding. Third, strong outbreeders become selfers during 

 senility; strong inbreeders undergo autogamy during senility. 

 Nevertheless, some degree of outbreeding is compatible with the 

 occurrence of autogamy. This is true because autogamy is never 

 followed by a period of immaturity. Hence, the homozygosity 

 resulting from autogamy can at once be compensated by restor- 

 ing heterozygosity through conjugation within the population. 

 The episode of autogamy serves to prevent the progression of 

 senility and to restore vigor, and it can do this without undue 

 sacrifice of heterozygosity, thanks to the clever device of sup- 

 pressing immaturity after autogamy. 



This summary of the life features that affect the breeding sys- 

 tem has thus far failed to refer to what might, a priori, have been 

 supposed to be the most important single feature, namely, the 

 mating types and their inheritance. In spite of the large amount 

 ol information available on this complex subject, I find it the most 

 difficult one to relate to breeding systems. Before attempting to 

 do so, the units with which wc shall have to deal will be re- 

 viewed. The basic unit is the caryonide, the group of individuals 



