206 PROTOZOA 



detail by considering the bearing of varietal differences on breed- 

 ing systems and evolution. 



Varietal Differences in Breeding Systems. As just mentioned, 

 most of the varietal differences are related to their breeding sys- 

 tems. Aside from this relation, they would appear, as they have 

 until now, as mere chaotic brute facts of nature. Recognition of 

 their relation to breeding systems makes them fit together, like 

 the pieces of a jigsaw puzzle, into a meaningful picture. The 

 nature of the relations will be best illustrated by contrasting first 

 the most extreme inbreeders with the most extreme outbreeders, 

 even though less is known about them than about some of the 

 less extreme varieties. I shall then review the situation in the in- 

 termediate varieties progressing from those that appear to be 

 least, to those that appear to be most, committed to inbreeding. 



1. The extreme inbreeders, varieties 10 and 14. These two va- 

 rieties have been found only once each in nature, in the south- 

 ernmost part of the United States. They are both apparently rare 

 and highly localized. This alone would limit narrowly their pos- 

 sibilities for cross breeding. Everything else that is known about 

 them points in the same direction. They have no detectable im- 

 mature period and the shortest known mature period which ex- 

 tends to about the 15th fission after conjugation or autogamy. 

 This means two things: they are able to cross-conjugate immedi- 

 ately after they complete the reorganization following fertiliza- 

 tion; but only for a short time thereafter. After that the response 

 to depletion of food is autogamy, the closest possible form of 

 inbreeding. 



In estimating the chances for cross conjugation during the 

 short period in which it is possible, several considerations are 

 pertinent. The processes of meiosis and fertilization and the sub- 

 sequent process of nuclear reorganization are relatively rapid in 

 these varieties, and they afford little time for dispersal while they 

 arc in progress. The organisms occur in warm regions, and this 

 also hastens the processes. The fission rates are among the highest, 

 and this would lead to the most rapid exhaustion of available 

 food and cut down on time available for migration before star- 

 vation .main leads to fertilization. These varieties belong to group 



