Although we have no good evidence of the existence in organic beings of an innate tendency towards progressive development, yet this necessarily follows, as I have attempted to show in the fourth chapter, through the continued action of natural selection. For the best definition which has ever been given of a high standard of organisation, is the degree to which the parts have been specialised or differentiated; and natural selection tends towards this end, inasmuch as the parts are thus enabled to perform their functions more efficiently.
Izaki organikoek garapen hazkorrerako sortzetiko joerarik duten froga baliozkorik ez dugun arren, hori, hala ere, laugarren kapituluan frogatzen saiatu naizen moduan, hautespen naturalaren ekintza jarraikiaren berezko ondorioa da. Izan ere, goi-mailako organismo ereduaz eman den definiziorik onena, organoek lortu duten espezializazio edo eranizte maila da, eta hautespen naturalak xede horrexetara jotzen du, era horretan oraganoei beren eginkizunak efikazia handiagoz betetzeko ahalmena ematen zaielako.