The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility, both of first crosses and of hybrids, is simply incidental or dependent on unknown differences in their reproductive systems; the differences being of so peculiar and limited a nature, that, in reciprocal crosses between the same two species, the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction.
Aipaturiko arau eta egitateok, aitzitik, uste dut argiro adierazten dutela bai lehen gurutzaketen bai hibridoen antzutasuna intzidental hutsa dela edo beren ugal aparatuetako ezberdintasun ezezagunen mende dagoela, eta, ezberdintasunok hain izaera bereizi eta mugatukoak direlarik, espezie bereko bi banakoren elkarrekiko gurutzaketetan, bataren sexu-elementu arrak eragozpenik gabe ihardunen duen sarri bestearen sexu-elementu emearengan, baina ez alderantziz.