But not only the age, but every temporary ebb of the vital force or other disturbance of health in the parents at the time when the child is begotten may interfere with the part of one or other, and prevent the appearance of a man of eminent talent, which is therefore so exceedingly rare a phenomenon. It may be said, in passing, that in the case of twins the absence of all the differences just mentioned is the cause of the quasi‐identity of their nature. If single cases should be found in which a highly gifted son had a mother who was not mentally distinguished at all, this may be explained from the fact that this mother herself had a phlegmatic father, and on this account her more than ordinarily developed brain was not adequately excited by a corresponding energy of the circulation—a necessary condition, as I have explained above in chapter 31. Nevertheless, her highly perfected nervous and cerebral system was transmitted to the son, in whose case a father with a lively and passionate disposition and an energetic action of the heart was added, and thus the other physical condition of great mental power first appeared here. Perhaps this was Byron’s case, since we nowhere find the mental advantages of his mother mentioned. The same explanation is also to be applied to the case in which the mother of a son of genius who was herself distinguished for mental gifts had a mother who was by no means clever, for the father of the latter has been a man of a phlegmatic disposition. The inharmonious, disproportionate, ambiguous element in the character of most men might perhaps be referred to the fact that the individual has not a simple origin, but derives the will from the father and the intellect from the mother. The more heterogeneous and ill‐adapted to each other the two parents were, the greater will that want of harmony, that inner variance, be. While some excel through their heart and others through their head, there are still others whose excellence lies in a certain harmony and unity of the whole nature, which arises from the fact that in them heart and head are so thoroughly adapted that they mutually support and advance each other; which leads us to assume that the parents were peculiarly suited to each other, and agreed in an exceptional measure.