Not only the condition of imperfect mental development, but also that of inability to withstand stress upon the nervous system, may be inherited. From the studies of Dr. Rosanoff and his collaborators, it appears that if both parents be subject to manic depressive insanity or to dementia precox, all children will be neuropathic also; that if one parent be affected and come from a weak strain, half of the children are liable to go insane; and that nervous breakdowns of these types never occur if both parents be of sound stock. Finally, a study of families with special abilities reveals a method of inheritance quite like that of nervous defect. If both parents be color artists or have a high grade of vocal ability or are littérateurs of high grade, then all of their children tend to be of high grade also. If one parent has high ability, while the other has low ability but has ancestry with high ability, part of the children will have high ability and part low. It seems like an extraordinary conclusion that high ability is inherited as though due to the absence of a determiner in the same way as feeble-mindedness and insanity are inherited. We are reminded of the poet: "Great wits to madness sure are near allied." Evidence for the relationship is given by pedigrees of men of genius that often show the combination of ability and insanity. May it not be that just that lack of control that permits "flights of the imagination" is related to the flightiness characteristic of those with mental weakness or defect? These studies of inheritance of mental defect inevitably raise the question how to eliminate the mentally defective. This is a matter of great importance because, on the one hand, it is now coming to be recognized that mental defect is at the bottom of most of our social problems. Extreme alcoholism is usually a consequence of a mental make-up in which self-control of the appetite for liquor is lacking. Pauperism is a consequence of mental defects that make the pauper incapable of holding his own in the world's competition. Sex immorality in either sex is commonly due to a certain inability to appreciate consequences, to visualize the inevitableness of cause and effect, combined sometimes with a sex-hyperesthesia and lack of self-control. Criminality in its worst forms is similarly due to a lack of appreciation of or receptivity to moral ideas. If we seek to know what is the origin of these defects, we must admit that it is very ancient. They are probably derived from our ape-like ancestors, in which they were _normal_ traits. There occurs in man a strain that has not yet acquired those traits of inhibition that characterized the more highly developed civilized persons. The evidence for this is that, as far back as we go, we still trace back the black thread of defective heredity.