A Theory by Which to Work

Darwin always read widely, on the lookout for new ideas. In late September 1838 he found himself reading--"for amusement," he later recalled--the "Essay on Population" by political economist Reverend Thomas Malthus. In this essay, Malthus argued that human population could quickly outstrip the food supply: competition for food or space was a constant force keeping population in check.

Darwin immediately saw how the idea could be applied to the natural world. More animals were born than could survive. They constantly struggled against one another for food or room to grow, he thought. That meant any plant or animal with a competitive edge--drought tolerance, a thicker-than-average coat--could live longer and leave more offspring than its fellows. The presence of such adaptations controlled, in effect, which individuals would represent the species in the next generation.

Now Darwin could see how variation could make a difference: Individuals with useful traits would, on average, survive to reproduce and pass along those traits. "It is a contest, " he wrote, and "a grain of sand turns the balance."

Three Principals

Charles Darwin's "D Notebook"
 October 1838--July 1839


Darwin's D Notebook.

©AMNH / Denis Finnin

The D Notebook is fourth and last in the series of transmutation notebooks. In it Darwin considers such subjects as adaptation--the fit between organisms and their place in nature--and the pace of geological change. Perhaps most important, in this notebook he spells out the theory of natural selection.

"Three principles will account for all
 (1) Grandchildren. like. grandfathers
 (2) Tendency to small change. . especially with physical change
 (3) Great fertility in proportion to support of parents"

Darwin's last principle refers to Malthusian population pressure. He wrote this entry sometime between late November and early December 1838.

Notebook courtesy The Syndics of Cambridge University Library