Borrowing from Thomas Malthus’s principle of population growth, Darwin argues that species must engage in a struggle for existence that limits their possibilities for survival. Malthus shows that while population growth occurs exponentially (doubling every generation until it reaches infinite numbers), geography and limited natural resources (food, water, shelter) restrict the number of organisms that are able to survive in nature. Since the earth cannot accommodate an infinite number of inhabitants, species must compete with one another to survive.

This competition creates what Darwin calls the “struggle for existence,” in which species must fight to survive to avoid extinction. Any small advantage that an organism might have—the ability to run away quickly from predators, coloring that allows it to hide from predators, physical adaptations that allow it to gather food more easily—will give it an edge over other organisms, bettering its odds of winning the struggle over others and surviving.

Darwin identifies two ways in which species can win the struggle for existence. First, they can simply survive and live longer than other organisms. Second, they can leave more offspring than other species do, ensuring that their traits will be passed on to subsequent generations more frequently than will the traits of organisms that reproduce less. These methods of perpetuation are interrelated: An organism that survives over others is also more likely to produce more offspring in its longer span of life. The struggle for existence, then, is not just about individual survival; it is about the survival of groups of related organisms over other groups. Organisms are related through the variations they share, and it is the advantages these variations give to the group that allow them to survive over other organisms.