What else should he do? It is remarkable that Fisher did not consider the possibility of analyzing the logically simplest case of one sex (which could equally well be viewed as the case of no sexes, the absence of any differentiation in gender). Yet, this seems to be the most basic problem we face, when we want to understand the evolution and occurrence of different sexes: why is the simplest thinkable sexual system, namely a population in which all gametes are equivalent and every gamete can mate with any other gamete, not realized in Nature?