A particular motion, originated in the ovary and in the testis, accumulates such energy in the nervous centers as eventually to bring the masculine element in contact with the feminine, so that the generative gemmulæ produced in the slow laboratory of two different organisms reunite in that nest which is the maternal womb and where the fecundated egg must transform into a human being. The poet and the metaphysician may define love in whatever manner they choose. There is only one definition for science: Love is the energy which must bring in contact the egg with the seed; without ovary and without testis there can be no love. That forward movement which is called generation is so powerful as to oppose and even destroy the minor motion, that is, the preservation of the individual; and while each individual rotates, it is carried forward with a movement a hundred times more irresistible and powerful through space and time. The first motion represents the narrow life of the individual and is protected by egotism; the second is the great life of the species, and love defends it. The most superficial study of the generative function is sufficient to convince us that love is always a phenomenon of high chemistry, in which the generating atoms, in order to combine, must be neither too similar nor too dissimilar. No sooner has sex manifested itself in animals than we have in the same individual, but in two distinct laboratories, the formation of two generative elements. Sex, which, at first thought, appears to us as one of the deepest mysteries of life, is nothing but a laboratory which attracts the elements generated by every element of the organism, and encloses and preserves them in itself in order to pour them into the bosom of other elements, similar but not equal, generated in another laboratory, that is, the opposite sex. When the two generative laboratories are separated in two distinct organisms, it is probable that the diversity of their gemmulæ is greater. If in individuals closely resembling each other, but of different races, we combine the generative elements, we still will probably have fecundity; while, if we pass to different species, fecundity will be more difficult; if we pass to different genera it will in most cases become impossible.