Why capable of reproducing fertile offspring is the logical Darwinian definition of same species

In the logic of Darwin's natural selection system , to be selected for a species , a phenotype , a genotype, must be perpetuated through the generations . In a sexually reproducing species , this means that two individual organisms, one female and one male are the same species, if they mate and produce fertile offspring. Only members of a species are capable of perpetuating the species and having it selected for . So, this is the logical Darwinian definition of species or membership in the same species . So, an animal species is a population of females and males capable of mating and producing fertile offspring . Who ever heard of a one generation species ? Charles Brown