Slide showing a chart demonstrating 'testis size and hybrids between race A and race B of drosophila pseudoobscura'. Slide shows a chart demonstrating 'testis size and hybrids between race A and race B of drosophila pseudoobscura'. The chart consists of 16 sets of chromosomes on the Y axis, and the X axis shows 'length of the testis in micra from 0 to 700'. Title below image reads: 'Fig 21 - testis size and hybrids between race A and race B of Drosophila pseudoobscura. Chromosomes of race A are represented white; those of race B black (from Dobzhansky).' Late 19th to early 20th century