In order that any great amount of modification should be effected in a species, a variety, when once formed must again, perhaps after a long interval of time, vary or present individual differences of the same favourable nature as before; and these must again be preserved, and so onward, step by step.
Espezie bati eraldaketa handiren bat sor dakion, jada formaturiko aldaki batek, agian denboraldi luze baten ondoren, berriro beste aldakuntza bat ukan edo lehengo moduko alde indibidual faboragarria agertu beharra du, eta hauek ere iraun beharra dute, eta horrelaxe gero ere urratsez urrats.