It is certain, on the one hand, that the sterility of various species when crossed is so different in degree and graduates away so insensibly, and, on the other hand, that the fertility of pure species is so easily affected by various circumstances, that for all practical purposes it is most difficult to say where perfect fertility ends and sterility begins.
Egia da, alde batetik, hainbat espezieren antzutasuna oso maila ezberdinekoa dela eta oso mailaketa sentigaitza erakusten duela eta, bestetik, espezie garbien ugalkortasunak oso aise jasotzen duela inguruabar ezberdinen eragina, eta horregatik, xede praktiko guztietarako, arrunt zaila da esatea ugalkortasun hobezina non bukatzen den eta antzutasuna non hasten den.