The gist (in brief) of Darwin - Wallace theory is as follows. (i) Individuals within species show considerable but continuous variation in the form and physiology. (ii) This variation arises in a random fashion and is heritable. (iii) The potential for increase within population of animals and plants is considerable. (iv) since resources are limited, so individuals in a population struggle for their own existence. (v) Only some survive and leave offsprings with the same trait - through this natural selection of the fittest species become represented by individuals which are better adapted.