but with respect to the more important and adaptive characters, the passage from one stage of difference to another may be safely attributed to the cumulative action of natural selection, hereafter to be explained, and to the effects of the increased use or disuse of parts.
baina ezaugarririk garrantzitsuen eta moldakorrenei dagokienez, ezberdintasun gradu batetik besterako iragaitza, gero azalduko dugun hautespen naturalaren ekin metatuari eta organoen erabilera edo erabilerarik ez hazkorraren eraginari zor izan dakieke.