PROTOZOA 



single pond. Pringle's data also throw some light on what goes on 

 1 the range occupied by variety 9. The pertinent data appear in 

 his table that gives information on segregation of serotype alleles 

 in the F2 by autogamy after crosses between homozygotes for 

 different alleles. Here the proportion of nonviable F2 clones is 

 the significant information. A cross between a Scottish and 

 French strain gave 16.9% F2 mortality. Crosses between Scottish 

 strains from different sources gave 7 to 20% F2 mortality, mean 

 13.4%. Crosses between different isolates from the one intensively 

 studied pond gave 2 to 10% F2 mortality, mean 6.3%. As far as 

 they go, these data indicate that F2 mortality is less following 

 conjugation within a local population than following conjugation 

 between different local populations. This implies to a certain de- 

 gree that inbreeding has a selective advantage over outbreeding. 

 But the impediment to outbreeding imposed by F2 mortality 

 is much less in this material than in some varieties to be dis- 

 cussed later. Taking all the facts into consideration, variety 9 ap- 

 pears to inbreed to a marked degree, even within a single local 

 population; but it is capable of and apparently undergoes some 

 small measure of outbreeding, at least within the confines of a 

 local population. 



Varieties 1, 2, and 3 show several features in common. All of 

 them show variations among strains with respect to the occur- 

 rence and duration of the immature period, from none at all up 

 to about a week or more. There is also some variation in the 

 duration of the moderate mature period, up to about 20 days or 

 about 75 fissions. These periods appear to be somewhat shorter 

 in variety 2. All undergo autogamy when starved during senility. 

 These common features indicate that inbreeding and outbreeding 

 occur to different extents in different strains of each of these 

 varieties. The main variable involved is the immature period. In 

 its absence, inbreeding is favored; in its presence, outbreeding is 

 favored in proportion to its duration. Each of these varieties thus 

 appears to be in process of evolutionary divergence with respect 

 to its breeding system and it may become necessary to subdivide 

 each variety into groups of strains differing with respect to the 



