Whatever the cause may be of each slight difference between the offspring and their parents-and a cause for each must exist-we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species.
Gurasoen eta ondorengoen arteko alde txikien kausa zeinahi dela ere-eta bakoitzak bere kausa izan behar du-, badugu oinarririk uste izateko ezberdintasun faboragarrien metaketa jarraikia dela espezie bakoitzaren azturekiko erlazioan egiturako eraldaketa garrantzizkoenei sorrera eman diena.