In considering the probability of natural selection having come into action, in rendering species mutually sterile, the greatest difficulty will be found to lie in the existence of many graduated steps, from slightly lessened fertility to absolute sterility.
Espezieak elkarrekiko antzu bihurtzeko badaitekeela hautespen naturalak jokatu izana kontuan hartzean, zailtasunik handiena hurrenez hurreneko maila asko izatean datzala aurkituko da, ugalkortasun pixka bat murriztutik erabateko antzutasuneraino.