Franklin's interest in these questions was civic as well as scientific. Then, as now, population was relevant to many aspects of society: laws governing reproduction; budgets for schools and hospitals; wage rates and job distribution; and the movement of people within and between nations.
As a public intellectual who consciously contributed to academic controversies and public policy, Franklin pursued these questions with vigor. In 1751, he put down his thoughts in the form of a small treatise: Observations Concerning the Increase of Mankind, Peopling of Countries, etc. The basis of his argument was breathtakingly simple: "There is ... no Bound to the prolific Nature of Plants or Animals, but what is made by their crowding and interfering with each other's Means of Subsistence."
Franklin argued that custom and law play only minor roles in population size. In the end, it's determined by the available means of subsistence.
Based on this insight, Franklin made two predictions that proved remarkably accurate: The population of North America would double every 25 years, and it would outstrip the population of Great Britain within 100 years.
(In early drafts, Franklin also worried about the "swarm" of German immigrants who threatened to make Pennsylvania "a colony of Aliens." These sharp words cost him an election, but they were deleted from editions published in England, and so they are not part of this story.)
The impact of Franklin's argument was profound. Writers ranging from Adam Smith and Samuel Johnson to Richard Price and William Godwin embraced and duplicated his key claims. Franklin's most influential reader was Thomas Robert Malthus, a young minister in the Church of England who worried that population always increases faster than the means of subsistence. As Malthus famously put it, population increases geometrically, while the means of subsistence increase only arithmetically.
The second edition of Malthus' Essay on the Principle of Population (1802) was a monumental work, more than 200,000 words long. Malthus announced his theme on the first page: The greatest obstacle to human happiness is "the constant tendency in all animated life to increase beyond the nourishment prepared for it. It is observed by Dr. Franklin, that there is no bound to the prolific nature of plants or animals, but what is made by their crowding and interfering with each other's means of subsistence. ... This is incontrovertibly true."
Malthus was criticized for his bleak view of the world. But his intellectual vigor, combined with his strong views on matters such as family planning, meant that he could not be ignored. So it was that, in 1838, a young English naturalist struggling to make sense of observations made while surveying the Galapagos Islands started to read Malthus' Essay.
As Darwin later recalled, his observation of plants and animals had prepared him "to appreciate the struggle for existence." It struck him upon reading Malthus that "favorable variations would tend to be preserved and unfavorable ones to be destroyed. The result of this would be the formulation of new species." The theory of natural selection was born.
Darwin had strong family ties to Franklin. His grandfather Erasmus Darwin, a free-thinking physician and early evolutionist, was a friend and colleague of Franklin's during the American's long stays in England. Erasmus' son Robert - Charles' father - visited Franklin in Paris in the early 1780s. As a boy, Charles was treated to stories of Franklin's extraordinary wit.
In spite of these ties, though, we have no evidence that Darwin read Franklin's Observations. Yet a crucial paragraph of what's known as Darwin's Notebook D - the very place where modern scholars locate the first clear formulation of the doctrine of natural selection - begins with Malthus' statement of the relationship between population and subsistence.
The concept of conflict between geometric and arithmetic growth belonged to Malthus. But the core ideas that population is limited only by the available means of subsistence, and that the natural rate of increase for humans was 25 years, were Franklin's.
Franklin cherished improvements to the world. Near the end of his life, he regretted having been "born so soon." Were Franklin alive today, he would surely take delight in the thought that, in fathering an evolution of science, he had also grandfathered the science of evolution.
Alan Houston is a political science professor at the University of California, San Diego.
Alan Houston is the author of "Benjamin Franklin and the Politics of Improvement"
(Yale University Press).