Ploss, Das Weib, Vol. I, chap. XX.

It must not, however, be assumed that the rural immigrants are in the mass better suited to urban life than the urban natives. It is probable that, notwithstanding their energy and robustness, the immigrants are less suited to urban conditions than the natives. Consequently a process of selection takes place among the immigrants, and the survivors become, as it were, immunized to the poisons of urban life. But this immunization is by no means necessarily associated with any high degree of nervous vigour or general physical development.

Havelock Ellis, A Study of British Genius, pp. 22, 43.