With hermaphrodite organisms which cross only occasionally, and likewise with animals which unite for each birth, but which wander little and can increase at a rapid rate, a new and improved variety might be quickly formed on any one spot, and might there maintain itself in a body and afterward spread, so that the individuals of the new variety would chiefly cross together.
Noizean behin besterik gurutzatzen ez diren organismo hermafroditetan, eta kume bakoitzerako batzen diren baina hara eta hona askorik ibili ez eta azkar ugaldu ahal diren animalien artean ere, behingoan era daiteke edozein tokitan aldaki berri eta hobetu bat, eta talde bat eginez iraun eta barreiatu, eta horren kariaz, aldaki berriko banakoek batez ere elkarrekin gurutzatu beharra izanen dute.