The Origin of Species: Chapter 3 Summary & Analysis

Darwin wrote about his intention to examine how the struggle for existence affected natural selection. He believed that because of each species’ struggle to survive, even light variations in a species help the offspring to survive. This process of preserving small, helpful variations is natural selection.

The Term, Struggle for Existence, used in a large sense. Darwin clarified that when he used the term “struggle for existence,” he meant it not just in the literal sense but also in the metaphorical sense. He clarified that he didn’t just mean dogs literally fighting over food, but also cases like desert plants that “struggle” to get enough moisture.

Geometrical Ratio of Increase. Darwin noted that every organism encounters some form of destruction in its lifespan. Otherwise, its population would increase geometrically and quickly outgrow available resources, similar to what Thomas Malthus wrote about. Darwin had evidence from humans and domestic animals of just how quickly populations could increase under favorable conditions. But while many plants and animals have the potential to grow this rapidly, it is rarely the case. Factors like food scarcity and climate all contribute to “destruction” that prevents rapid, endless increases.

Nature of the Checks to Increase. Darwin admitted that the “checks” which stopped a population from increasing at a rapid rate were not always clear or easy to determine. The food supply is one obvious, extreme limit, but more often the size of a species is limited by predators. Climate also restricts population growth, with extreme cold and droughts being particularly effective at checking growth.

When a species experiences very favorable conditions and grows rapidly in a small area, it often faces other checks, such as an epidemic, that help keep the population size under control. In other cases, a species population must be large in order to survive its predators—for example, corn must produce a lot of seeds to account for how many seeds get eaten by birds.

Complex Relations of all Animals and Plants to each other in the Struggle for Existence. Darwin noted how the populations of plants and animals are often checked through their relationships with each other. For example, in some areas, cattle stop Scotch fir trees from growing because they eat the saplings, but once the cattle are enclosed, the Scotch firs spring up. These relationships between species can take many forms and be affected by other factors like geography, climate, and seasons.

Struggle for Life most severe between Individuals and Varieties of the same Species. Darwin asserted that the more similar species are, the more directly they will be in competition with each other. Even very different species can have similarities, however, and the claws of a tiger are not so different from the claws of a parasite riding on the tiger.

Darwin suggested that his audience should try to imagine what could happen to give one species an advantage over another. He argued that every living thing is struggling to try to increase its population at a rapid rate, but that in the pursuit of this, it will also experience occasional destruction. While this could seem dark, Darwin took it as a consolation that ultimately “the vigorous, the healthy, and the happy survive and multiply.”