The struggle for existence

Darwin came upon the explanation while reading Malthus's An Essay on the Principle of Population. He continued:

"In October 1838, that is fifteen months after I had begun my systematic enquiry, I happened to read for amusement 'Malthus on population', and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favorable variations would tend to be preserved and unfavorable ones to be destroyed. The result of this would be the formation of a new species."

Because of the struggle for existence, forms that are better adapted to survive will leave more offspring and automatically increase in frequency from one generation to the next.

As the environment changes through time (for example, from humid to arid) different forms of a species will be better adapted to it than were the forms in the past; the better adapted forms will increase in frequency, and the now poorly adapted forms will decrease in frequency.

As the process continues, eventually (in Darwin's words) "the result of this would be the formation of a new species".