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 selectio