Darwin's book, *On the Origin of Species*, suggested that organisms evolve through a process called natural selection.

Here's how it works:

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

2. Inheritance: These variations can be passed down from parents to offspring.

3. Overproduction: Organisms produce more offspring than can survive.

4. Competition: The offspring compete for limited resources like food, water, shelter, and mates.

5. Survival of the fittest: Individuals with traits that make them better adapted to their environment are more likely to survive and reproduce, passing on those advantageous traits.

This process, acting over many generations, leads to changes in the characteristics of a population, with organisms becoming better suited to their environment.