Crossing

Abstract

An examination of the causes for variability, for geno-variation, shows that there are two out-standing ones, spontaneous geno-variation or mutation, and the recombination of genes through crossing in the widest sense. We will examine mutation as a cause for that geno-variation which can furnish the material for evolution, and see that, as there is a good reason to assume that mutation ever consists of anything but the spontaneous loss of a gene, it cannot be considered as of any real general importance for the evolution of new species. This brings us to the question whether crossing, re-combination of genes made possible by heterozygosis, is in itself sufficient to cause the genotypix variation which is necessary for evolution.