The geologists of Darwin's time were revising previous estimates of Earth's age. Some suggested that Earth may be millions of years old, whereas previous assumptions were that it was only thousands of years old.
Why was an older estimate of Earth's age important to Darwin's theory?
Darwin observed evidence of migrations between continents, which would take long periods of time to complete.
Darwin proposed natural selection as a slow and gradual process.
DNA mutations are rare occurrences.
Extinctions are rare events, but there is evidence of many extinct organisms.