ABSTRACT
A species now undergoing subdivision is Drosophila pseudoobscura. There are two races, A and B which differ from each other, not in any observable external appearance but in the arrangement of genes on the chromosomes. In another species of Drosophila, melanogaster, character differences have appeared before there has been any reduction of fertility between members of the species. In nature, a number of species of Drosophila actually differ in the ways seen in the flies already discussed. The source of structures as complex as the eye has always been difficult to explain. The only alternative seems to be the assumption that at some time in the dim past, the chance association of the requisite chemicals in the presence of favorable temperature, moisture, etc., produced living protoplasm. It is possible that the problem of life’s beginning on our planet will always remain insoluble, a philosophical question rather than a subject capable of experimental investigation and solution.
