Animals having both sexes, do not necessarily show sexual dimorphism. But neither this exact likeness of individuals, nor the double function with which they are charged, contradicts the general law which seems to wish that an individual should be due to elements coming from two different individuals. Autofecundation is exceptional, is very rare. Whether or no the individual possess the two genital glands, or one of them only, it needs a male, or an individual acting as male, and a female or an individual acting as female, to perpetuate life. Alternative hermaphrodism confirms these propositions, be it that the same gland transforms itself totally, turn by turn, into male principle, then into female principle; be it divided between a male half and a female half, these two halves ripen simultaneously or successively. When there is total or partial alternation, the male principle is ready first, and waits: thus the aggressivity of the male, and the passivity of the female are visible in the most obscure manifestations of sexual life: the fundamental psychology of an ascide does not differ from that of an insect, or from that of a mammal. CHAPTER XIII THE MECHANISM OF LOVE _V. Artificial fecundation.--Disjunction of the secreting apparatus from the copulating apparatus.--Spiders.--Discovery of their copulative method.--Brutality of the female.--Habits of the epeire.--The argyronete.--The tarantula.--Exceptions: the reapers.--Dragon-flies (libellule).--Dragon-flies (demoiselle) virgins and "jouvencelle."--Picture of their love affairs._ The apparatus for secreting sperm and that for copulating are sometimes separated. The female has a vagina normally situated; the male has no penis, or else it is situated in some part of the body not in symmetry with the receiving apparatus. It is then necessary either for the male to make an artificial penis, as one has seen in the cephalopodes, and as in the spider, or for him to engage in complicated manœuvres to dominate the female, and to engineer the conjunction of the two apparatus, as does the dragon-fly (libellule).