There is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature, than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent their inarching in our forests.
Espezieak, gurutzaketak eta naturan nahastea eragozteko, antzutasun maila ezberdinez bereziki hornituak izan direla pentsatzeko ez dago arrazoi gehiagorik, beste hau pentsatzeko baino, alegia, zuhaitzak gure basoetan hurbilketaz txerta daitezen eragozteko, txertatzeko zailtasun maila ezberdin eta zerbait antzekoz hornituak izan direla.