In the two previous lectures of this course the subject of “Variation in its Relation to the Origin of Physical Malformations, of Congenital Mental Defect, and of the Neuroses, such as Epilepsy, Hysteria, and Alcoholism,” was considered. The correlation between congenital malformation and congenital mental defect was pointed out. The relation of the neuroses to one another, their heredity, and their distribution throughout mankind of all races, was insisted on. It was also shown that all these affections are genetic in origin and independent of so-called causes or influences due to the environment. For the environment is not, and cannot be, constant while the manifestations in question are, so far as we know, universal. It was further shown that congenital malformation and congenital mental defect are due to inherent processes, the nature of which is at present unknown, acting within the fertilised ovum. It is, moreover, certain that these processes must be independent of the environment of the elements contributed by either parent, of the immediate state of health of the parents or, with certain exceptions such as injuries or special disease, of the uterine environment. It is not asserted that diseases affecting the mother or even, on rare occasions, specific affections of the father may not deleteriously influence the growing embryo in utero. The fact remains, however, that in the majority of instances these defects are hereditary, that they may pass over several members of the same family, and in the case of animals over several members of the same litter—nay, even that they pass over one or more generations to re-appear in a succeeding one. In face of such facts, it is useless to speculate upon physical causes while the great innate cause remains obscure.