The consideration that the wild population is immensely larger than the numbers bred experimentally even in Drosophila, and that the duration of the species is still more disproportionate to the experimental periods, leads to the conclusion that the majority of the mutations which we can hope to observe in cultures must have occurred previously with very great total frequency in the wild species. It may be calculated that with mutation rates of the order of 1 in a million the corresponding selection in the state of nature, though extremely slow, can not Safely be neglected in the case of heterozygotes. The observed behavior of multiple allelomorphs largely supports, though that of specific modifiers seems to oppose, the view that complete dominance generally may be regarded as a product of such selective modification.