For this reason reversions to remote ancestors occur readily when species and breeds are crossed, while they are rare in the normal inbreeding of a species. The reversions of the breeds of pigeon to their wild ancestral form, the slaty-blue rock-pigeon, never result, as Darwin showed, and as we have already noticed, from pure breeding of one race, but only when two or more breeds are repeatedly crossed with one another. Even then it occurs by no means always, but only now and again. The germ-plasm of the breeds must therefore still contain ids of the rock-pigeon, but in a small number, varying from individual to individual. If by fortunate reducing divisions and the meeting together of a sperm-cell rich in ancestral ids with a similarly endowed egg-cell the number of ancestral ids be raised to such a point that it exceeds the number of modern-breed ids contained in each one of the conjugating germ-cells, the ancestral ids control the development and reversion occurs, for the ancestral ids together have a cumulative effect while the ids of the two parent breeds are different and therefore, as far as they are so, cannot be co-operative in their influence. But it must be understood that they need not be different as far as _all_ their determinants are concerned, but usually only as regards some groups, and thus it happens that reversion does not occur in regard to all, but only in regard to particular characters--thus in the _Datura_ hybrids, chiefly in regard to the colour of the flowers and the stem, and in the hybrids between different breeds of pigeon mainly in regard to the colour and marking of the plumage. The reversions of the horse and ass to striped ancestors, which Darwin has made famous, go much further back into the ancestral history of the species, for while we know the ancestral form of the domestic pigeon in the still living rock-dove (_Columba livia_), the ancestral form common to the horse and the ass is extinct, and we can only suppose that it was striped like a zebra, because such striping occasionally occurs in pure horses and pure asses, at least in their youth, although now only on the legs, and because this striping is often more marked in the hybrid between the horse and the ass, the mule. In Italy, where one sees hundreds of mules, the striping is not exactly frequent, but it may occur in about two per cent., while in America it is said to be much more frequent. The germ-plasm of the horse and the ass must therefore contain, in varying numbers, ids whose skin-colour determinants represent in part still unmodified ancestral characters. When two germ-cells chance to meet in fertilization, both of which have received, through a favourable reducing division, a relatively large number of such ids, a relative majority of these in the fertilized ovum is opposed to the dissimilar and therefore mutually neutralizing homologous determinants of horse and ass, and reversion to the ancestral form occurs.