He who will explain in a definite manner why, for instance, an elephant or a fox will not breed under confinement in its native country, whilst the domestic pig or dog will breed freely under the most diversified conditions, will at the same time be able to give a definite answer to the question why two distinct species, when crossed, as well as their hybrid offspring, are generally rendered more or less sterile, while two domesticated varieties when crossed and their mongrel offspring are perfectly fertile.
Zehatz-mehatz azaldu dezakeenak, adibidez, elefantea edo azeria zergatik ez diren ugaltzen beren berezko herrialdean hesipeturik bizi direnean, etxe-txerria edo txakurra, ostera, baldintzarik ezberdinduenetan eragozpenik gabe umatzen diren artean, horrek zehatz esan ahal izanen du beste hau ere, alegia, zergatik gertatzen diren antzu bi espezie ezberdin, gurutzatzen direnean, eta era berean haien ondorengo hibridoak ere, bi aldaki etxekotu, guratzatzen direnean, eta haien ondorengo mestizuak, ostera, erabat ugalkorrak diren artean.