Most modern accounts see him as a gentle genius ahead of his time, a reluctant visionary set apart from contemporary ideas about biological transformation. But some historians question these popular versions of Darwin' s life and thought. The profundity of Darwin's theory of evolution by natural selection is belied by its apparent simplicity. The message Darwin took from Malthus' Essay on the Principle of Population was that creatures have far more progeny than can possibly survive, given the finite resources available. This leads to competition among the progeny for resources.
Progeny vary slightly from their parents and from one another.
Survivorship would be determined by the suitability of a given variety to the prevailing environmental circumstances. As the environment changed, so would the varieties most suited to its demands. Over time, the proportions of different varieties in a species would change, and one species would change, very slowly, into another. Lyell showed that the age of the Earth could be measured in hundreds of millions of years. Even though we now know this to have been an underestimate by an order of magnitude, it is still very much older than the few thousand years suggested in the Bible.