Another consideration, which disqualifies the factorial mutant for the rôle of a new species, is its failure to pass the test of interspecific sterility. When individuals from two distinct species are crossed, the offspring of the cross is either completely sterile, as instanced in the mule, or at least partially so. But when, for example, the sepia-eyed mutant of the vinegar fly is back-crossed with the red-eyed wild type, whence it originally sprang, the product of the cross is a red-eyed hybrid, which is perfectly fertile with other sepia-wild hybrids, with wild flies, and with sepia mutants. This proves that the sepia-eyed mutant has departed, so to speak, only a varietal, and not a specific, distance away from the parent stock. Ordinary or factorial mutation does not, therefore, as De Vries imagined, produce new species. These mutants do, indeed, meet the requirement of permanent transmissibility, for their distinctive characters cannot be obliterated by any amount of crossing. Nevertheless, the factorial mutation falls short of being an empirical proof of evolution, because it is a varietal, and not a specific, change. In other words, factorial mutants are new varieties and not new species. Only a heritable change based on germinal acquisition of sufficient magnitude to produce gametic incompatibility between the variant and the parent type would constitute direct evidence of the transmutation of species, provided, of course, that the variant were also capable of survival under the natural conditions of the wild state. In his Toronto address of December 28, 1921, Wm. Bateson announced the failure of De Vries’ Mutation Theory, when he said: “But that particular and essential bit of the theory of evolution, which is concerned with the origin and nature of species remains utterly mysterious. We no longer feel as we used to do, that the process of variation, now contemporaneously occurring, is the beginning of a work which needs merely the element of time for its completion; for even time cannot complete that which has not yet begun. The conclusion in which we were brought up that species are a product of a summation of variations ignored the chief attribute of species first pointed out by John Ray that the product of their crosses is frequently sterile in greater or less degree. Huxley, very early in the debate, pointed out this grave defect in the evidence, but before breeding researches had been made on a large scale no one felt the objection to be serious. Extended work might be trusted to supply the deficiency. It has not done so, and the significance of the negative evidence can no longer be denied....