Darwin’s theory of natural selection was influenced by the works of Thomas Malthus, an English political economist. In his “An Essay on the Principle of Population”, Thomas Malthus asserts that there are two fixed laws in nature: “food is necessary to the existence of man” and “the passion between the sexes is necessary and will remain nearly in its present state.” (Malthus 39) Malthus theorized that population increases in a “geometrical ratio” while the resources for subsistence increase in a “linear ratio.” (Malthus 39) Consequently, mathematical principles reveal that the geometrical growth of the human population will rapidly surpass the available resources. Malthus further claims that the two disparate forces of population and resources must be balanced and maintained at fairly equal levels. All species, plants and animals, have a natural tendency to increase their numbers through reproduction. In order to implement a balance between reproduction and resources, there must be natural checks on population such as “waste of seed, sickness, and premature death” among plants animals, and “misery and vice” among humankind (Darwin 40). In The Origin of Species, Darwin applies Malthusian principles to all species rather than just humankind and Malthusian logic serves as the principal basis for his monumental theory of natural selection.
In The Origin of Species, Darwin discusses the variation of species and notes that there is greater variation in domesticated species when compared to wild species of nature. He then examines the conditions that contribute to variation, such as the struggle for existence. Darwin states that the struggle for existence is an unavoidable consequence of the “high geometrical powers of increase” as described by Malthus (Darwin 97). However, rather than restricting the Malthusian idea of geometrical increase to human populations, Darwin extends it to all species of the animal and plant kingdoms because all organic beings have the potential to increase exponentially in the absence of a carrying capacity and limiting factors such as predation, disease, and limitation of resources. According to Darwin, the struggle for existence is ultimately the severe competition that all organic beings are exposed to due to a limitation of resources that can’t sustain all produced individuals.
Inherently, the purpose of all organic beings is to survive and to increase in numbers by leaving progeny. As a law of nature, most organisms are inclined to increase in a geometrical ratio, because most animals breed and most plants produce seeds. Although the rates of increase may differ, all organisms increase in numbers high enough to inundate the entire planet with their single species (Darwin 109). Darwin presents several examples to mathematically demonstrate his ideas. Darwin states that “even slow-breeding man has doubled in twenty-five years, and at this rate, in a few thousand years, there would literally not be standing room for his progeny” (Darwin 109). In addition, Linnaeus calculated that if a plant produced only two seeds annually, in twenty years the original plant would lead to the production of a million plants. Lastly, Darwin presents the example of the elephant. If an elephant produces three offspring throughout its lifetime, there will be fifteen million elephants produced by the end of the fifth century. In addition to theoretical calculations, there have been actual cases in nature in which populations have increased immensely under certain circumstances such as favorable environmental conditions and seasons. Similarly, when species are placed in new environments, they may increase exponentially due the absence of their natural predators. In these cases, the populations exhibit a geometrical rate of increase and lead to overpopulation. Furthermore, organisms have developed evolutionary strategies that allow them to maximize the number of offspring that survive. In unstable environments where there is great destruction in an organism’s early life, individuals produce a large number of eggs to ensure that at least some survive. In comparison, in stable environments where individuals are able to protect their young, organisms produce a small number of eggs which they can protect and secure their survival (Darwin 110).
If populations were to increase in a geometrical manner, the planet would not be able to sustain all of its inhabitants. Malthus firmly believed that “human species would increase in the ratio of-1,2, 4, 8, 16, 32, 64, 128, 256, 512, &c. And subsistence as-1, 2, 3, 4, 5, 6, 7, 8, 9, 10, &c. In two centuries and a quarter, the population would be to the means of subsistence as 512 to 10, in three centuries as 4096 to 13, and in two thousand years the difference would be almost incalculable, though the produce in that time would have increased to an immense extent.” (Malthus 40) Therefore, Malthus concludes that dangerous population growth must be hindered by limiting factors and certain measures such as late marriage or no marriage, and birth control. However, Darwin believes that all organic beings must battle with limiting factors and destructive forces. “It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase in food, and no prudential restraint from marriage.” (Darwin 97). Individuals must struggle for existence either within the same species, with the members of other species, or with external environmental conditions. Darwin compares the force of nature to a “yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.” (Darwin 111) Therefore, natural mechanisms exist to impose checks on all populations rather than the socially instituted, preventative checks Malthus discusses such as restriction of marriage and childbirth.
An organism’s struggle for existence is an essential part of Darwin’s explanation regarding variation and the creation of different species. Darwin’s theory is based on the observation that there are slight variations among individuals and there are more organisms produced than can possibly survive. Due to the production of more individuals than can survive, there is competition for resources and a struggle for existence resulting in only few individuals that can survive. Therefore, any variation that will provide the slightest advantage which will enable individuals to survive and better adapt to their environments will be selected for. These traits will be passed on to the offspring so that the offspring will have a better chance at survival as well. The process of selection and inheritance will continue for generations after generations, producing new species, and rendering other species to extinction. The theory of natural selection is the cornerstone of Darwin’s The Origin of Species.
Malthus’ “An Essay on the Principle of Population” provided Darwin a rationale for intraspecific and interspecific competition, the competition that exists within species and between species. Darwin used the Malthusian logic as a basis for his theory regarding the struggle for existence and natural selection. Despite its influence on Darwin’s theory of natural selection, Malthus is heavily criticized for his extremely gloomy views regarding human population growth. Malthus failed to predict the technological and agricultural advances that revolutionized the production of resources and food supplies that helped sustain the growing population. Malthus extrapolated a population explosion that would eradicate all of the natural resources of the planet and insisted that draconian laws must be imposed on humans in order to keep the population in check. The modern world today serves as an indication that predictions of Malthus were erroneous. However, the planet may be nearing its carrying capacity and there is no certainty that it will be able to sustain a growing population. Currently there are constraints on the world population with global warming, widespread extinction of plant and animal species, decreases in global food production, loss of ecosystems, and wider spread of infectious diseases. Although humankind may not completely destroy the planet and its resources completely, it is expected that world population will level off and most demographers expect that fertility rates will eventually decline below replacement ending the population explosion (Newbold 31).