T. M. SONNEBORN 175 



small a fraction we have learned about the complexity, variety 

 and marvels of this "simple, primitive, stereotyped animal." Yet 

 there is no escaping the task of trying to comprehend the situa- 

 tion if we are to attempt to reduce it to significant order in rela- 

 tion to species problems. I shall therefore take up the various 

 differences among the varieties in this section and shall try to 

 reduce them to significant order in the next section. 



Mating Type Specificity. The definite and sharp distinctions 

 among the mating types uniquely mark off each variety, as 

 described in the preceding section. No further comment is 

 needed here. Toward the end of this section, the usefulness of 

 these distinctions in identification will be considered. 



Distribution. The distributions of the varieties in the major 

 land masses and according to temperature were stated in the 

 preceding section. The usefulness of this knowledge in relation to 

 identification is limited of course by the inadequacy of the sam- 

 pling which underlies our present knowledge. With due allow- 

 ance for this, the source of a culture still has suggestive value 

 in identification. For example, a sample from northern Europe 

 in all probability would be found to belong to variety 1, 2, or 9, 

 and there is no use considering other possibilities until these have 

 been tested. Such considerations save much time in identifica- 

 tion, but seldom if ever could a strain be reliably identified solely 

 on the basis of its natural source. 



Temperature in Relation to Survival and Fission Rate. The 

 characteristic distribution of each variety with respect to tem- 

 perature suggests the existence of varietal differences in tem- 

 perature tolerance. Very little is known about this. Sonneborn 

 and Dippell (unpublished data of 1942) found that strain P of 

 variety 1 from Maryland grew well at 36.4°, but the strains of 

 the other four varieties (2, 3, 4, and 7) tested did not survive at 

 this temperature very long. Of the latter, variety 3 (strain 58 

 from Indiana) and variety 7 (strain 40 from Florida) grew well 

 at 34°; but variety 2 (strain 50 from Oregon) and variety 4 

 (strains 29 from Maryland and 51 from Indiana) did not survive 

 at 34°. These results indicate that predictions based on geograph- 

 ical distribution cannot be relied upon. For varieties 3 and 7 



