Like Charles Darwin and Alfred Russel Wallace, he read Malthus' "Essay on the principle of population", and was deeply impressed by its argument. Malthus said that sooner or later population growth gets checked by famine and disease, and there are then many deaths. So, he was prepared for Darwin's idea of natural selection, which he preferred to call "survival of the fittest". He did this in "Principles of Biology" (1864), after reading Charles Darwin's "On the Origin of Species".