Very little knowledge exists concerning the effect of the crossing of races in man. On the whole it appears not to be favourable, if it is a question of crossing of races from far apart, even in purely physical respects. An example of harmful influence is given in v. d. Velden's Table C 117--"+Fertility and Health in relation to the crossings of races+."
The next and the greatest concern of race-hygiene--much greater than the relative increase of inferiority--is, to-day, neomalthusianism, the intentional restriction of the number of births in varying degrees up to complete unproductiveness. Though conscious regulation of the production of children is absolutely necessary, it becomes fatal to a nation if under no control but the egotism of the individual. For its permanent prosperity a nation requires, in order merely to hold its own, a sufficient number of "hands" and a sufficient number of "heads" to guide those "hands." We referred to this when mention was made of sterility as a phenomenon of degeneration, but this cause of sterility during the last decades only takes a second place compared to deliberate intention. The wealthy and higher social classes were first attacked by neomalthusianism. Their progeny is becoming more and more utterly insufficient, so that under our present social conditions, particularly which give mind and talent better openings, and thereby more and more take out of the mass of the people the better elements, make the strongest demand for them and use them up, the danger of an increasing deterioration of the average quality of its progeny grows greater and greater. The baneful influence of wealth on fertility is shown by several tables. Figure C 118 "+Fertility and Wealth+," after Goldstein and Tallquist, gives the condition in the French Departments; Figure C 119, "+Number of Children and Wealth+," after Bertillon, for the Arrondissements of Paris; Figure C 120, "+Fertility and Wealth+," after Mombert, for Münich, 1901, Table C 121, "+The Number of Children in Families of Different Classes in Denmark+, 1901," after Westergaard; Table C 122, "+Fertility of Marriages, Occupation, and Wealth for Copenhagen, and Dutch Conditions+," after Rubin, Westergaard, and Verrijn Stuart.