On the Origin of Species | Study Guide

Charles Darwin

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On the Origin of Species | Chapter 3 : Struggle for Existence | Summary



After attempting to further define the term species, Darwin moves on to what he sees as the driving force behind the natural selection of wild animals—the struggle for existence. He states Herbert Spencer's term, survival of the fittest, can be applied to this concept. It is simply the struggle for existence as it affects an organism and their potential for offspring, and thus affects a species on a population level.

Darwin argues the struggle for existence stems from the geometrical rate of population increase. Thomas Robert Malthus first applied this idea to human populations, and Darwin argues mechanisms to keep population growth in check must exist. Otherwise, populations will outpace the environment's capacity to support them.

Darwin describes different methods that keep a population in check: the propensity for death in young or juveniles; the ability to obtain food, or being food for another species; climatic effects; and epidemics. He emphasizes in general, the environment is sustained by a series of checks and balances. He illustrates this principle using changes in the ecological composition of a heath (an open area of uncultivated land) following the introduction of one new tree species—the Scotch fir. He notes massive changes in both the floral and faunal communities, and goes on to marvel at individuals who are surprised when creatures go extinct. Equally, Darwin posits domesticated animals may be unable to survive if returned to their wild habitat.

Darwin ends with an important observation—the struggle for existence influences the natural selection of wild species. "The struggle will generally be more severe between" species within the same genera than between species from different genera. It is this competition that results in subtle variations, which lead to the formation of distinct species.


One of the statements most often associated with Darwin today is natural selection is the product of the "survival of the fittest." However, it was Herbert Spencer, a 19th-century British philosopher, who originally coined the phrase. The term is often misunderstood as survival of the fittest individual. However, when applied to the theory of natural selection, it refers to the survival of the fittest on the population or species level.

Darwin's argument for why the struggle for existence occurs is based on the work of Thomas Robert Malthus, an English economist who theorized population growth will outpace the available food supply. His ideas were published in 1798 in An Essay on the Principle of Population. While Malthus argued for population control to limit the pressures of human populations, Darwin argues for the presence of natural population controls as it pertains to animal populations.

Darwin's description of the changes to a heath due to the introduction of the Scotch fir is important because of the implications of his observations. Today, ecologists and scientists know the ecology of a specific region is immensely sensitive to subtle shifts in the floral or faunal population. This is one of the major issues plant and animal populations are facing today due to climatic changes. As the temperature of the earth changes, plants and animals that adapted to a specific region are being placed in conditions that do not fit their evolutionary adaptations. As a result, they are no longer able to exist in that region. In contrast to other contemporary scientists, Darwin's recognition that life is highly susceptible to extinction is a fact that even today, portions of the general public struggle to understand.

Equally, Darwin's comment regarding the potential inability of domesticated animals to survive in their wild habitats has since proven to be true. For instance, while this is not the case for all domesticated animals, domesticated cows would suffer if left without farmers, due to the selection for their overproduction of milk.

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