Where nothing prevents gametes of 2 spp. from uniting, interspecific sterility rests on the failure of the hybrid zygote to develop normally. Such failure occurs either in initial stages of development or at maturity. In the latter case degeneration of reproductive cells before and after maturation, as well as disturbed meiosis, may be responsible for sterility. Although single genes that condition sterility are known and data are available on the effect of ploidy and chromosomal rearrangements on the fertility of hybrid offspring, it is clear that complete interspecific sterility has a multi-factorial basis and must have evolved slowly by the accumulation of genie differences. Unbalanced hybrid zygotes would finally add their isolating influence to the process of divergent evolution, thus making interspecific sterility a by-product rather than a point of departure in evolution.