To return, however, to the metista, the horse (_Equus caballus_) and the ass (_Equus asinus_) represent two distinct species under a common genus. This is indicated by the fact that the mule, which is the hybrid offspring of their cross, is entirely sterile, producing no offspring whatever, when mated with ass, horse, or mule. Such total sterility, however, is not essential to the proof of specific differentiation; it suffices that the hybrid be less fertile than its parents. As early as 1686, sterility (total or partial) of the hybrid was laid down by John Ray as the fundamental criterion of specific distinction. Hence Bateson complains that Darwinian philosophy flagrantly “ignored the chief attribute of species first pointed out by John Ray that the product of their crosses is frequently sterile in a greater or lesser degree.” (_Science_, Jan. 20, 1922, p. 58.) Accordingly, the sameness of type required in members of the same species refers rather to the genotype, that is, the sum-total of internal hereditary factors latent in the germ, than to the phenotype, that is, the expressed somatic characters, viz. the color, structure, size, weight, and all other perceptible properties, in terms of which a given plant or animal is described. Thus it sometimes happens that two distinct species, like the pear-tree and the apple-tree, resemble each other more closely, as regards their external or somatic characters, than two varieties belonging to one and the same species. Nevertheless, the pear-tree and the apple-tree are so unlike in their germinal (genetic) composition that they cannot even be crossed. According to all theories of transformism, new species arise through the transformation of old species, and hence evolutionists are at one in affirming the occurrence of specific change. When it comes, however, to assigning the agencies or factors, which are supposed to have brought about this transmutation of organic species, there is a wide divergence of opinion. The older systems of transformism, namely, Lamarckism and Darwinism, ascribed the modification of organic species to the operation of the external factors of the environment, while the later school of orthogenesis attributed it to the exclusive operation of factors residing within the organism itself.