Charles Darwin as a tremendously influential nineteenth century biologist, known primarily for his 1859 book On the Origin of Species. In this book, he argues that more complex species gradually evolved from less complex ones over millions of years. He suggests two major mechanisms for this process of evolution, random mutation and natural selection. Mutations, according to Darwin, are not adaptive, but random variations. Over time, individuals with favourable mutations are more likely to survive to reproductive age and rear young successfully, and thus mutations gradually propagate through a population. This process of "survival of the fittest" or "Natural selection" accounts for evolution.