One of the most prominent objections to the selection theory, which strikes us at once from the standpoint of natural history, is the following: The varieties of a domesticated species, obtained by artificial breeding, are lost, and return to the original wild form of the species as soon as they are crossed long enough with other varieties or are left to themselves and to the crossing with individuals of the original form of their species; and hence we can not see how individual characteristics, even if favorable to the individual, will not be lost again by the crossing which is inevitable in a state of nature, with such individuals as do not possess those characteristics. Besides, it is an established fact, confirmed by all our observations stretching over thousands of years, that the characteristics of species are preserved in spite of all individual modifications, and that this preservation of the characteristics of species has its cause essentially in the free crossing of individuals. This objection induced Moriz Wagner to take up again an idea already expressed by Leopold von Buch, and to complete the principle of a selection through natural breeding by another, and partly, indeed, to supplant it by the principle of _isolation by migration_. Isolated individuals, who, from any reason naturally to {53} be accounted for, leave the mass of their fellows, can from the very consequence of this isolation transmit to their offspring common individual characteristics which are not destroyed again by the crossing with other individuals. They will especially fix and transmit these individual characteristics, when they are favorable to them for the conditions of existence in their new place of living, and these individual characteristics will so much the more be increased and developed in a direction favorable to the subsistence of the individuals in their new place of living, as there are more closely connected with this isolation variations in the conditions of existence, in climate, geographical surroundings, food, and so on. He very attractively applies this theory also to the explanation of the origin of man. According to his opinion, even the nearest animal progenitors of man were isolated, and the isolating power was the rise of the great mountains of the Old World, which took place previous to the glacial period. One pair, or perhaps a few pairs, of those progenitors were driven away from the luxurious climate of the torrid zone to the northern half of the globe, and found their return cut off by glaciers and high mountains; in place of a comfortable life on the trees, necessity urged them to gain support from conditions less favorable to existence, and necessity, this mother of so many virtues and achievements, finally made man what he is. In following out these ideas, Moriz Wagner has gradually and more and more decidedly given up the selection theory, and opposed it by sharp criticisms.