What three main sources did Charles Darwin use in formulating his theory of evolution?

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I would say that Darwin's idea was original with him. As a young man, he was initially inspired by reading Lyell's Principles of Geology as well as Thomas Malthus's writings on overpopulation in humans. Not sure what third source your teacher (?) is asking about. Darwin was extremely well read and also spent years interviewing breeders in order to understand artificial selection. So he talked to people who bred cows, horses, pigeons, dogs, etc. Plus he was continually classifying, performing small experiments, thinking about everything all the time, and taking notes on all his ideas in journals.

ROLCAM, Are you taking a take home exam?

asking,

I am very interested in discussing these subjects.
My next birthday will be my 80th.

Just my views:-

Darwin used three principal sources in developing his theory. These were (1) his personal observations, (2) the geological theory of the British scientist Sir Charles Lyell, and (3) the population theory of the British economist Thomas Robert Malthus. Darwin made many of his observations as a member of a scientific expedition aboard the H.M.S. Beagle from 1831 to 1836. The ship made stops along the coast of South America, and Darwin collected many specimens of plants and animals and wrote detailed notes.

Hi, ROLCAM!

As far as finding the three main sources as to how Darwin formulated his theory of evolution, I would think that you've already found them.

Is there something else, concerning this, that you'd like to discuss, please?

Thanks!