PROTOZOA 



widespread varieties of group B give no sexual reactions with 

 each other. Of the varieties of group B with more limited distri- 

 bution, variety 12 has been found only once (in Madagascar) 

 and it seems to be very different from the other group B varieties. 

 The three remaining varieties, 8, 10, and 14, have been found 

 only in North America, almost exclusively in its southernmost 

 parts. As has been pointed out in detail, these three varieties are 

 much alike in a number of ways and are also much like variety 4. 

 These resemblances and distributions suggest that varieties 8, 10, 

 and 14 evolved relatively recently from variety 4 directly or 

 indirectly. 



3. Gaps in knowledge. Many of the gaps in present knowl- 

 edge have already been mentioned. A most important one is the 

 paucity of information on the large animal varieties, both those 

 that form two micronuclei and two macronuclei after fertilization 

 and those that form four of each. Whether the latter constitute 

 a distinct and readily recognizable species (P. multimicronucle- 

 atum) or whether they intergrade into the others or cannot be 

 readily identified is one problem that needs solving. 2 Another is 

 the possibility of mating and of serologic relations with the small 

 animal varieties, a subject barely touched as yet. A third problem 

 concerns how many varieties of large animals occur and what 

 their distributions are. Attention is just beginning to be directed 

 to this. When these matters are cleared up, a good deal more 

 about evolution in these organisms will doubtless become appar- 

 ent and some of the inferences in the preceding section may well 

 have to be changed. It may, for example, turn out that certain 

 large animal varieties are the closest to the ancestors of both 

 group A and group B. I have long suspected, on a priori grounds, 

 l hat the ancestors of these groups were more extreme outbreeders 

 than any small animal variety yet found; and some of the large 

 animal varieties are extreme outbreeders. 



Isolating Median is ins. Geographic isolation appears to be the 

 most potent isolating mechanism in P. aurclia. Although it is 

 obvious that the organisms have become spread over the face of 

 the earth, the means by which they accomplish this remains un- 



