T. M. SONNEBORN 231 



8 is thus attested by both their serologic and their mating type 

 specificities. The only sexual reactions between groups A and B 

 involve variety 8 of group B. It reacts and mates with variety 3 

 and gives barely perceptible reactions but no mating with varie- 

 ties 1 and 7. Sexual reactions between complementary mating 

 types of different varieties, without regard to their intensity, occur 

 in 6 of 42 possible combinations within group A, 6 of 42 within 

 group B, and 4 of 84 between groups A and B. They are thus three 

 times as frequent within each group as between groups. If only 

 those that lead to mating are counted, there are 6 among the 84 

 possibilities within groups, and only 1 of the 84 possibilities be- 

 tween groups. The mating type specificities are thus on the whole 

 less different within a group than between groups. 



What light, if any, does the geographical distribution of the 

 varieties throw on their evolution? On this subject, knowledge 

 is of course meager, but it is suggestive. In group A, only variety 

 1 is known to be cosmopolitan; variety 9 has been found only in 

 Europe, varieties 3, 5, 7, and 11 only in North America. Variety 

 15 will be disregarded for little is known about it. These facts of 

 distribution suggest that varieties 3, 5, 7, 9, and 11 may have 

 arisen from variety 1. The antiquity of variety 1 is indicated by 

 its occurrence in Australia and Hawaii as well as in both Ameri- 

 cas, Asia, and Europe. The relative recency of origin of the others 

 is suggested by the fact that each is, so far as now known, con- 

 fined to a single continent. And the close relationship of varieties 

 3, 5, and 7 to variety 1 is indicated by sexual reactions that occur 

 between them. Serological evidence relates varieties 3 and 9 to 

 variety 1. Significantly, varieties 3 and 5 are localized near the 

 northern, and variety 7 near the southern limit of the range of 

 variety 1 in North America. Altogether it seems reasonable to 

 conclude that the other group A varieties (3, 5, 7, and 9) arose 

 from variety 1 relatively recently. 



Several varieties of group B have a wide distribution, but only 

 variety 2 is cosmopolitan. Variety 6 occurs in India, Puerto Rico, 

 and southernmost North America and it may be assumed to be 

 subtropical around the world. Variety 4 has been found in Aus- 

 tralia, Japan, and both Americas, but not in Europe. These three 



