As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be NATURALLY SELECTED. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.
Espezie bakoitzetik biziraun ahal izanen duen baino askoz ere banako gehiago jaiotzen denez, eta, ondorioz, behin eta berriro gertatzen den existentziaren aldeko borroka dagoenez gero, hemendik dator izaki orok, baldin bizi-baldintza konplexu eta noizbehinka aldakorren pean beretzat mesedegarri den edozein eratan aldatzen bada, denik arinkien izanik ere, bizirauteko eta modu horretan naturalki hautetsia izateko ahalbide gehiago izatea.