In the supplements to the second book the will was compared to the root and the intellect to the crown of the tree; and this is the case inwardly or psychologically. But outwardly or physiologically the genitals are the root and the head the crown. The nourishing part is certainly not the genitals, but the villi of the intestines: yet not the latter but the former are the root; because through them the individual is connected with the species in which it is rooted. For physically the individual is a production of the species, metaphysically a more or less perfect picture of the Idea, which, in the form of time, exhibits itself as species. In agreement with the relation expressed here, the greatest vitality, and also the decrepitude of the brain and the genital organs, is simultaneous and stands in connection. The sexual impulse is to be regarded as the inner life of the tree (the species) upon which the life of the individual grows, like a leaf that is nourished by the tree, and assists in nourishing the tree; this is why that impulse is so strong, and springs from the depths of our nature. To castrate an individual means to cut him off from the tree of the species upon which he grows, and thus severed, leave him to wither: hence the degradation of his mental and physical powers. That the service of the species, _i.e._, fecundation, is followed in the case of every animal individual by momentary exhaustion and debility of all the powers, and in the case of most insects indeed by speedy death, on account of which Celsus said, “_Seminis emissio est partis animæ jactura_;” that in the case of man the extinction of the generative power shows that the individual approaches death; that excessive use of this power at every age shortens life, while, on the other hand, temperance in this respect increases all the powers, and especially the muscular powers, on which account it was part of the training of the Greek athletes; that the same restraint lengthens the life of the insect even to the following spring; all this points to the fact that the life of the individual is at bottom only borrowed from the species, and that all vital force is, as it were, force of the species restricted by being dammed up. But this is to be explained from the fact that the metaphysical substratum of life reveals itself directly in the species and only by means of this in the individual. Accordingly the Lingam with the Yoni, as the symbol of the species and its immortality, is worshipped in India, and, as the counterpoise of death, is ascribed as an attribute to the very divinity who presides over death, Siva.