160 PROTOZOA 



rain are matched against the mating types of any other strain, 

 two very different results may be obtained. On the one hand, 

 the two strains may contain clones manifesting identical sets of 

 mating types. When this is so, the same rules of mating apply to 

 combinations of clones from different strains that apply to com- 

 binations of clones from the same strain : conjugation occurs if the 

 clones combined are of complementary mating types, not if they 

 are of the same mating type. 



On the other hand, two strains may manifest quite different 

 sets of mating types. Then no conjugation will occur when any 

 clone of the one strain is combined with any clone of the other 

 strain. Thus, conjugation does not result from mere difference 

 of mating type, but from complementariness of mating type. 

 There are multiple sets of complementary mating types within a 

 single species. Some strains have one set and these interbreed 

 freely. Other strains have another set, and these also interbreed 

 freely. But strains with the first set of types are sexually isolated 

 from strains with the second set of types. When many strains 

 of a species are examined, a number of such groups of strains is 

 discovered. As a rule, each is sexuallv isolated from all the 

 others. The mating types of one set usually will have nothing to 

 do with mating types of the other sets. 



From these results, it at once became evident that P. aurelia 

 and P. bursaria each consisted of a number of "biological spe- 

 cies." Both Jennings and Sonneborn were well aware of this and 

 considered the problem in their first papers of 1938, which re- 

 ported the basic facts. However, they were then unable to de- 

 scribe these biological species so that they could be readily dis- 

 tinguished and identified. They therefore agreed, for the time 

 being, to refrain from assigning specific names. Instead, they 

 designated the sexually isolated groups of strains within a spe- 

 cies as varieties, assigning to each variety a number. This usage 

 has persisted and spread, to the annoyance of some geneticists 

 and doubtless to the relief of protozoologists. Tn this paper, I 

 shall, among other things, inquire exhaustively — in view of sub- 

 sequent additions to knowledge — into the questions of whether 

 the time has come to recognize the varieties as species and of 



