That crossing tends to produce reversion is a fact familiar to breeders and fanciers, and one which is emphasized by Darwin. When pigeons are crossed, there is a strong tendency to revert to the slatey-blue tint and black bars of the ancestral rock-pigeon. There is always a tendency in sheep to revert to a black colour, and this tendency is emphasized when different breeds are crossed. The crossing of the several equine species (horse, ass, etc.) "tends in a marked manner to cause stripes to appear on various parts of the body, especially on the legs," and this _may be_ a reversion to the condition of a striped and zebra-like ancestor. Professor Jaeger described a good case with pigs. "He crossed the Japanese, or masked breed, with the common German breed, and the offspring were intermediate in character. He then recrossed one of these mongrels with a pure Japanese, and in the litter thus produced one of the young resembled in all its characters a wild pig; it had a long snout and upright ears, and was striped on the back. It should be borne in mind that the young of the Japanese breed are not striped, and that they have a short muzzle and ears remarkably dependent."[EE] Darwin crossed a black Spanish cock with a white silk hen. One of the offspring almost exactly resembled the _Gallus bankiva_, the remote ancestor of the parents. Such cases would seem to show that in our domestic breeds ancestral traits lie latent. The crossing of distinct varieties may either neutralize the variations artificially selected, and thus allow the ancestral characters which have been masked by them to reappear; or they may allow the elements of the ancestral traits, long held apart in separate breeds by domestication, to recombine with the consequent emergence of the normal characters of the wild species. But, in truth, any attempted explanations of the facts are little better than guess-work. There are the facts. And the importance of crossing as a determining condition in domesticated animals should make us cautious in applying reversion, as it occurs in such cases, to wild species which live under more stable conditions where crossing is of rare occurrence. _The Origin of Variations._ The subject of the origin of variations is a difficult one, one concerning which comparatively little is known, and one on which I am not able to throw much light.