The two races of Drosophila pseudoobscura Frolova, known respectively as race A and race B, appear indistinguishable externally. And yet they are clearly separate, as manifested by the complete sterility of the male offspring from the intercrosses. Each race has a definite geographic distribution, and can be identified cytologically by the shape of the Ychromosome (Lancefield, '29, Dobzhansky and Boche, '33).
An analysis of the nature of the racial difference in this case must, then, turn largely to the physiological characteristics. Some exploratory studies of this nature have, indeed, established a number of suggestive facts. Lancefield ( '29) and Boche (unpublished) observed a partial sexual isolation between the races. Shapiro ('32) reports that oviposition begins in race A sooner after the emergence from the pupae than in race B. Poulson ( '34) found a shorter developmental period in A than in B, the difference being due almost exclusively to the duration of the pupal stage. The two races are, consequently, two different physiological systems.
The present paper furnishes information concerning the fecundity of the two races at different temperatures. Fe- cundity proves to be greater in A than in B at higher temperatures, while at lower ones the relation is reversed. The geographic distribution of the races seems to stand in a casual connection with this fact.