Darwin's book, "On the Origin of Species", suggested that organisms evolve through a process called natural selection. This process can be summarized as follows:

1. Variation: Individuals within a species exhibit variations in their traits.

2. Inheritance: These variations are, to some degree, heritable, meaning they can be passed on to offspring.

3. Competition: Organisms compete for limited resources such as food, shelter, and mates.

4. Survival of the Fittest: Individuals with traits that make them better suited to their environment are more likely to survive and reproduce.

5. Adaptation: Over time, advantageous traits become more common in the population, leading to adaptation of the species to its environment.

In essence, natural selection is a process where the environment "selects" for traits that increase an organism's chances of survival and reproduction. This leads to a gradual change in the characteristics of a population over generations, driving evolution.