“There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.” — Charles Darwin
Darwin has established that variation exists; now he introduces the force that acts upon it. Inspired by Thomas Malthus’s essay on population, Darwin reveals a profound truth: every species produces far more offspring than can possibly survive. This relentless pressure—the struggle for existence—is the engine that drives natural selection.
In 1838, Darwin read Malthus’s Essay on the Principle of Population, and the central argument struck him forcefully: populations tend to grow geometrically (exponentially), while resources grow only arithmetically (linearly). The inevitable result is competition, scarcity, and death.
Darwin uses this term in “a large and metaphorical sense, including dependence of one being on another, and including (which is more important) not only the life of the individual, but success in leaving progeny.”
The struggle is not always violent combat. It includes the plant struggling against drought, the seedling competing with others for light, the prey evading predators, and the parent successfully raising offspring. Any factor that affects survival or reproduction is part of the struggle.
Every species has the potential to multiply rapidly. Darwin calculates what would happen if this potential went unchecked:
The elephant is the slowest-breeding known animal. Assuming it breeds from age 30 to 90 and produces only 6 offspring total, Darwin calculates that from a single pair, there would be about 19 million elephants alive after 750 years.
Yet elephant populations remain roughly stable. What prevents this explosion? Death—countless elephants that could have been born, never survive to breed.
For faster-breeding organisms, the potential is staggering. A single pair of sparrows could theoretically produce millions of descendants within decades. Plants produce thousands of seeds; fish release millions of eggs. Yet populations remain roughly constant over time.
What prevents geometric increase? Darwin identifies multiple “checks”:
The precise checks vary by species and environment, but the result is universal: most individuals that could exist, don’t. Most eggs never hatch; most seeds never germinate; most young never reach maturity.
Darwin emphasizes that the web of ecological relationships is extraordinarily complex. Seemingly unrelated factors can have profound effects:
Red clover depends on humble-bees (bumblebees) for pollination. Humble-bee nests are destroyed by field mice. Cats kill field mice. Therefore, the abundance of cats in an area affects the abundance of red clover—an indirect relationship no one would suspect without careful study.
This interconnectedness means that a change in one species ripples through the entire community. The struggle for existence involves not just direct competition but a web of dependencies and effects.
A crucial insight: the struggle is most intense not between different kinds of organisms, but between the most similar ones.
Organisms that are similar require similar resources: the same food, the same shelter, the same conditions. Two closely related species, or two varieties of the same species, compete directly for exactly the same niche. One will often drive out the other.
“As species of the same genus have usually… some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.” — Charles Darwin
This principle will become crucial in the next chapter. If competition is fiercest among similar forms, then any variation that allows an organism to exploit a slightly different niche—to diverge from its relatives—confers a significant advantage.
Nature appears peaceful and harmonious to casual observation—but this is an illusion. Beneath the surface, a relentless culling is always occurring.
We see the surviving adults, the successful plants, the triumphant predators. We don’t see the millions of eggs that never hatched, the seedlings that withered in the shade, the weakened prey that was caught. Every organism we observe is a survivor—a winner in an invisible lottery of death.
This struggle, Darwin argues, is not cruel or wasteful—it is the creative force of nature. By eliminating the less fit and preserving the more fit, it shapes species over time, adapting them ever more precisely to their conditions of life.