Before applying the principles arrived at in the last chapter to organic beings in a state of nature, we must briefly discuss whether these latter are subject to any variation.
— from The Origin of Species by Means of Natural Selection Or, the Preservation of Favoured Races in the Struggle for Life, 6th Edition by Charles Darwin

It cannot be asserted that organic beings in a state of nature are subject to no variation; it cannot be proved that the amount of variation in the course of long ages is a limited quantity; no clear distinction has been, or can be, drawn between species and well-marked varieties.
— from The Origin of Species by Means of Natural Selection Or, the Preservation of Favoured Races in the Struggle for Life, 6th Edition by Charles Darwin

We now almost certainly know that organic beings in a state of nature may be modified in various definite ways by the conditions to which they have been long exposed, as in the case of the birds and other animals in the northern and southern United States, and of American trees in comparison with their representatives in Europe.
— from The Variation of Animals and Plants under Domestication by Charles Darwin

But it may be urged, granting that organic beings in a state of nature present some varieties,—that their organisation is in some slight degree plastic; granting that many animals and plants have varied greatly under domestication, and that man by his power of selection has gone on accumulating such variations until he has made strongly marked and firmly inherited races; granting all this, how, it may be asked, have species arisen in a state of nature?
— from The Variation of Animals and Plants under Domestication — Volume 1 by Charles Darwin

If then organic beings in a state of nature vary even in a slight degree, owing to changes in the surrounding conditions, of which we have abundant geological evidence, or from any other cause; if, in the long course of ages, inheritable variations ever arise in any way advantageous to any being under its excessively complex and changing relations of life; and it would be a strange fact if beneficial variations did never arise, seeing how many have arisen which man has taken advantage of for his own profit or pleasure; if then these contingencies ever occur, and I do not see how the probability of their occurrence can be doubted, then the severe and often-recurrent struggle for existence will determine that those variations, however slight, which are favourable shall be preserved or selected, and those which are unfavourable shall be destroyed.
— from The Variation of Animals and Plants under Domestication — Volume 1 by Charles Darwin