Concerning the causation of degeneracy, we may say, as of every aspect of personality, that its roots are to be looked for somewhere in the mingling of hereditary and social factors from which the individual life springs. Both of these factors exhibit marked variation; men differ in their natural traits very much as other animals do, and they also find themselves subject to the varying influences of a diversified social order. The actual divergences of character and conduct which they exhibit are due to the composition of these two variables into a third variable, the man himself. In some cases the hereditary factor is so clearly deficient as to make it natural and justifiable to regard heredity as the cause; in a much larger number of cases there is good reason to think that social conditions are more particularly to blame, and that the original hereditary outfit was fairly good. In a third class, the largest, perhaps, of all, it is practically impossible to discriminate between them. Indeed, it is always a loose way of speaking to set heredity and environment over against each other as separable forces, or to say that either one is the cause of character or of any personal trait. They have no separate existence after personal development is under way; each reacts upon the other, and every trait is due to their intimate union and co-operation. All we are justified in saying is that one or the other may be so aberrant as to demand our special attention. Congenital idiocy is regarded as hereditary degeneracy, because it is obvious that no social environment can make the individual other than deficient, and we must work upon heredity if we wish to prevent it. On the other hand, when we find that certain conditions, like residence in crowded parts of a city, are accompanied by the appearance of a large per cent. of criminality, among a population whom there is no reason to suppose naturally deficient, we are justified in saying that the causes of this degeneracy are social rather than hereditary. The fact probably is, in the latter case, that the criminality is due to the conjunction of degrading surroundings with a degree of hereditary deficiency that a better training would have rendered harmless, or at least inconspicuous; but, practically, if we wish to diminish this sort of degeneracy, we must work upon social conditions.