same family to effect fertilization. These variations may have arisen as a result of the continued inbreeding, because the same ids were constantly being brought together in the fertilized ova, and thus any unfavourable directions of variations which existed were strengthened. It seems to me that in this way alone can the injurious effect of inbreeding be made intelligible. From both parents identical ids meet in the fertilized ovum, in greater numbers the longer inbreeding continues, for at the maturation of every germ-cell the number of different ids is diminished by a few, and their number must therefore gradually decrease, and it is conceivable that ultimately it may sink to one kind of id, that is, that the germ-plasm may then consist entirely of identical ids. If chance variations of certain determinants in unfavourable directions occur in some of the ids composing the germ-plasm, these are brought together in the offspring from both the maternal and the paternal side, and will occur in an increasing number of ids the longer the inbreeding has gone on, that is, the smaller the number of different ids has become. The unfavourable variation-tendency is therefore persisted in, and its influence upon the development of a new descendant will be the greater the larger the number of identical ids with these unfavourable variations. It is obvious that the crossing of an animal, which is thus, so to speak, degenerating slightly, with a member of an unrelated family must immediately have a good effect upon the descendants, for in this way quite different ids with other variations of their determinants are introduced into the inbred germ-plasm which had become too monotonous. From this theoretical interpretation of the injurious consequences of inbreeding we may at once infer that not every inbreeding necessarily implies degeneration, for the occurrence of unfavourable variational tendencies in the germ-plasm is presupposed as the starting-point of degeneration, and if these do not exist there can be no degeneration. This harmonizes with the fact that the evil effects of inbreeding are observed _to vary greatly in amount, and may not occur at all_. But they are greatest in breeds artificially selected by man, which have long been under unnatural, directly influential conditions, and are also removed from the purifying influence of natural selection. In such cases, therefore, there is every probability that diverse unfavourable variational tendencies in the determinants will occur.