hand, characters acquired through inbreeding or other debilitating causes may disappear or become recessive as soon as crossing permits a return to a more normal and vigorous ancestral type of organization, as in the historic pigeon experiments of Darwin. The popularization of Mendel's laws should make it more easy to perceive that the normal effect of cross-breeding is a progressive synthetic evolution and not a stationary average, though we are having some fine examples of the lengths to which the specialist will sometimes go to escape facts too simple and obvious for his appreciation.
Individual 'Hybrids.'—Perhaps the loosest use of the word hybrid is for the offspring of crosses between so-called 'horticultural varieties' of domesticated plants propagated by cuttings or grafts. Everybody knows, though some forget, that the Baldwin apple, the Bartlett pear, the Niagara grape, and a great multitude of analogous sorts, are descending from single seedling trees or vines, and are thus for evolutionary purposes single individuals. The distinction between such individuals and those of wild species in nature is largely psychological; we have learned to regard differences between individual apple trees, but have not attained such close acquaintance with oaks and elms. If crosses between the normally diverse individuals of a species are to be termed hybrids then the word covers all sexually differentiated organisms and is utterly useless as a means of drawing biological distinctions. Mendel deliberately disregarded the question as to which of his pea hybrids were between different species, and which between varieties merely, and for the purposes of his inquiry this was a matter of little importance. But for his followers to draw general conclusions, while ignoring all distinction between the evolutionary conditions of the organisms which they study, is a reversion to the same general woolliness of evolutionary thinking to which Mendel constituted so brilliant an exception.
The millions of species with which nature has been experimenting for millions of years seem to make it very plain that individual diversity with free interbreeding is the optimum condition for evolutionary progress, since this is what we find everywhere among natural species. It is true that the diversity masks the slow and gradual motion of the species from perception by our momentary observations, and also that the interbreeding hinders the segregation of species; but we may take the results as evidence that evolutionary progress is not impeded by wide individual variation, nor by opportunities for the progressive accumulation of new characters. Nor need we turn our backs on this interpretation of the history of organic nature because Mendel and others have given new demonstrations of the old fact that there are degrees of evolutionary divergence in which the combination of parental characters is no longer possible.
