5. There is also a physiological distinction between species, namely, that the individuals are sterile with one another, whereas this does not apply to varieties; and though Darwin has labored to break down this distinction by insisting on rare exceptional cases, and suggesting many supposed ways by which varieties of the same species might possibly attain to this kind of distinctness, the difference still remains as a fact in nature; though one not readily available in practically distinguishing species. These general properties of species will, I think, be admitted by all naturalists as based on nature, and absolutely necessary to the existence of natural history as a science, independently of any hypotheses as to the possible changes of specific forms in the lapse of time. I now proceed to give a similar summary of the laws of the varieties which may exist--always be it observed, within the limits of the species. 1. The limits of variation are very different in different species. There are many in which no well-marked variations have been observed. There are others in which the variations are so marked that they have been divided, even by skilful naturalists, into distinct species or even genera. I do not here refer to differences of age and sex. These in many animals are so great that nothing but actual knowledge of the relation that subsists would prevent the individuals from being entirely separated from one another. I refer merely to the varieties that exist in adults of the same sex, including, however, those that depend on arrest of development, and thus make the adult of one variety resemble in some respects the young of another; as, for instance, in the hornless oxen, and beardless individuals among men. If we inquire as to the causes on which the greater or less disposition to vary depends, we must, in the first place, confess our ignorance, by saying that it appears to be in a great measure constitutional, or dependent on minute and as yet not distinctly appreciable structural, physiological, and psychical characters. Darwin states that Pallas long ago suggested, from the known facts that the seeds of hybrid plants and grafted trees are very variable, the theory that mixture of breeds tends to produce variability; but Darwin does not seem to attach much importance to this, and admits our inability to explain the origin of these differences.[161] We know, however, certain properties of species that are always or usually connected with great liability to variation. The principal of these are the following: 1. The liability to vary is, in many cases, not merely a specific peculiarity; it is often general in the members of a genus or family. Thus the cats, as a family, are little prone to vary; the wolves and foxes very much so. 2. Species that are very widely distributed over the earth's surface are usually very variable. In this case the capacity to vary probably adapts the creature to a great