According to our most conservative estimates, the introduction of the potato accounts for approximately one-quarter of the growth in Old World population and urbanization between 1700 and 1900.Of course the most famous aspect of the potato was the fact that the Irish relied upon a specific variety so heavily that when a fungus hit the island, the crops were wiped out, leading to a famine. This seems like a classic Malthusian dilemma, but of course Malthus died a decade and a half before that happened. Might he have thought such a thing possible? It turns out that 6th Edition of his book on Population is available on-line and a quick consultation turns up the fact that he already knew, early in the 19th century, how important the potato was to population growth in Ireland, and he worried that this was going to lead to early marriages, high birth rates, and overpopulation:
The details of the population of Ireland are but little known. I shall only observe therefore, that the extended use of potatoes has allowed of a very rapid increase of it during the last century. But the cheapness of this nourishing root, and the small piece of ground which, under this kind of cultivation, will in average years produce the food for a family, joined to the ignorance and depressed state of the people, which have prompted them to follow their inclinations with no other prospect than an immediate bare subsistence, have encouraged marriage to such a degree, that the population is pushed much beyond the industry and present resources of the country; and the consequence naturally is, that the lower classes of people are in the most impoverished and miserable state. The checks to the population are of course chiefly of the positive kind, and arise from the diseases occasioned by squalid poverty, by damp and wretched cabins, by bad and insufficient clothing, and occasional want. To these positive checks have, of late years, been added the vice and misery of intestine commotion, of civil war, and of martial law [Part II, Chapter X, paragraph 38].