Heredity is the chief cause of feeble-mindedness, and a normal child is never born of two feeble-minded parents. The very thorough investigation of the heredity of the feeble-minded which is now being carried on at the institution for their care at Vineland, New Jersey, shows even more decisive results. By making careful pedigrees of the families to which the inmates at Vineland belong it is seen that in a large proportion of cases feeble-mindedness is handed on from generation to generation, and is traceable through three generations, though it sometimes skips a generation. In one family of three hundred and nineteen persons, one hundred and nineteen were known to be feeble-minded, and only forty-two known to be normal. The families tended to be large, sometimes very large, most of them in many cases dying in infancy or growing up weak-minded.31
Not only is feeble-mindedness inherited, and to a much greater degree than has hitherto been suspected even by expert authorities, but the feeble-minded thus tend (though, as Davenport and Weeks have found, not invariably) to have a larger number of children than