What we really need to know is whether or not the sexual method was once acquired, because it was an advantage to a particular organism, or to the species to reproduce in this way. It is assumed by many writers that this was the case, but whether they have sufficient ground for forming such an opinion is our chief concern here. On the other hand, it is conceivable, at least, that if the sexual method once became established, it might continue without respect to any superiority it gave over other methods, and might finally become a necessary condition for the propagation of particular species. Thus the method would become essential to propagation without respect to whether the species lost more than it gained. Whichever way the balance should turn, it might make little difference, so long as the species was still able to propagate itself. Brooks made the interesting and ingenious suggestion that the separation of the sexes has been brought about as a sort of specialization of the individuals in two directions. The male cells are supposed to accumulate the newly acquired characters, and represent, therefore, the progressive element in evolution. The female cells are the conservative element, holding on to what has been gained in the past. It does not seem probable, in the light of more recent work, that this is the function of the two sexes, and it is unlikely that we could account for the origin of the two sexes through the supposed advantage that such a specialization might bring about. A number of writers, Galton, Van Beneden, Bütschli, Maupas, and others, have looked at the process of sexual reproduction as a sort of renewal of youth, or rejuvenescence of the individuals. There is certainly a good deal in the process to suggest that something of this sort takes place, although we must be on our guard against assuming that the rejuvenescence is anything more than the fulfilment of a necessary stage in the life history. Weismann has ridiculed this suggestion on the ground that it is inconceivable that two organisms, decrepit with old age, could renew their youth by uniting. Two spent rockets, he says, cannot be imagined to form a new one by combining. There is apparent soundness in this argument, if the implication is taken in a narrow physical sense. If, on the other hand, the egg is so constituted that at a certain stage in its development an outside change is required to introduce a new phase, then the conception of rejuvenescence does not appear in quite so absurd a light.