Fig. 57. Increase in frequency of chromosome type PP in the southwestern 

 United States populations of Dwsophila pseuoohscura. Above, samples made 

 during 1940 and earlier. Below, samples made in 1957. The relative proportion 

 of PP in the population is indicated by the size of the black circles. Although 

 the geographic scale is dififerent in the two maps, the scale of circle size is the 

 same and comparable in the two. (After Dobzhansky.) 



