T. M. SONNEBORN 237 



four varieties found by Hiwatashi to those lie had independently 

 found. Giese and Arkoosh (1939) found in western United States 

 a pair of interbreeding mating types which doubtless represented 

 a variety. Y. T. Chen (1944) found four varieties with two mat- 

 ing types each in China. Vivier (1955) found a variety with two 

 mating types in France. The relation of the varieties of Giese and 

 Arkoosh, of Chen, and of Vivier to those of Gilman has not been 

 reported. 



Of the 16 numbered varieties, 1 and 3 have been found in 

 Japan and the United States, 12 and 13 in Japan, 2 in North and 

 South America, 8 in North America and Scotland, 15 and 16 in 

 Europe, and the rest only in the United States. Most varieties 

 seem to be scarce or absent in warm regions, but varieties 1 and 

 3 and probably 12 and 13 appear to be concentrated in warm to 

 moderate regions. Two varieties may have a very restricted range 

 ( Gilman, 1956a and unpublished ) : variety 7 has been found only 

 on the island of Martha's Vineyard and neighboring Cape Cod; 

 variety 11 was found only once, in Connecticut. 



Varietal Differences. Varietal differences are being investi- 

 gated by Gilman (1941, 1956b), but relatively little has been 

 published on this subject. There are some differences in the tem- 

 peratures that give optimal mating reactions. Varieties 4 and 5 

 are resistant, while varieties 1, 2, and 3 are sensitive, to the killer 

 strain H of variety 2 of P. anrelia. The animals of variety 3 appear 

 to be smaller than those of certain other varieties. A number of 

 characteristic varietal differences in the dimensions of the animals 

 and of the micronuclei appear to exist, but most of the details are 

 yet to be published. Thus far, it would be quite impossible to 

 identify a variety by any features yet made known, except their 

 mating types. Hence, the situation is in general the same as in 

 P. aurelia with respect to the question of whether the varieties 

 should be given specific names. This would be quite useless. 



The Question of Gene Flow within and between Varieties. 

 Nine of the 16 varieties, which may be designated group 1, are 

 completely isolated from each other and from the other varieties 

 by inability to interbreed. Varieties 3 and 6, in one combination 

 of their mating types (VI with XI), give a faint sexual reaction 



