In order that an organic form be preserved, the individual must preserve itself and generate other individuals. Now these forces must vary inversely. If the individual, through its simple organization, is little fit to resist danger, it must countervail this weakness with reaction, generating intensely. If, on the contrary, high qualities give it a great capacity for self-protection, it should then diminish its fecundity proportionately. If danger is reckoned as a constant quantity, inasmuch as capacity for resistance should be equal in all species, and does consist of two factors (faculty to maintain individual life and power to multiply it), these factors cannot but vary in opposite directions. This most simple and sublime law, which Herbert Spencer read in the great book of nature, is one of those that rule with the most inflexible tyranny the elementary phenomena of reproduction, as well as the highest and most complex phenomena of human love. In the _Diatomaceæ_ the fecundity by scission is gigantic: Smith reckoned that a single gnat could create a thousand million individuals in one month. A young _Gonium_, capable of scission after twenty-four hours, can produce in a week 268,435,456 individuals equal to itself. In other cases, the process of multiplication is not scissiparous, but endogenous, as with the _Volvox_; but the reproduction is always extraordinary. If all the individuals generated should survive, a _Paramecium_ would, by scission, produce in the course of a month 268,000,000 individuals. Another microscopic animal can produce 170,000,000,000 individuals in four days. The _Gordius_--the entozoön of an insect--lays 8,000,000 eggs in less than a day. An African termite lays 80,000 eggs in twenty-four hours, and Eschricht reckoned at 64,000,000 the number of eggs in the adult female of an _Ascaris lumbricoides_. If, from the minute microscopic creatures exposed to every danger and which consume very little matter--if, from these living atoms of which you could gather as many in your hands as there are men on earth, you pass to the elephant, you have there a giant of flesh that requires thirty years of its life to become fecund, and then, after a long gestation, produces but one offspring. And above the elephant you find a giant of thought, Man, who requires the third part of his average life to reproduce himself, and after nine long months generates one child only; and, what is worse, he sees half of his offspring mowed down before they are able to bear flower and seed. The methods of transmitting life are manifold, since nature in no other function has been so inexhaustibly rich with forms as in generation; but we, dealing here with the general physiology of love, will reduce all the various generative forms to these few: