IF bears instead of men had been shipped from Europe to the North American continent, there would now be no more bears than in the time of Columbus, and possibly fewer, for bear food would not have been increased nor the conditions of bear life extended, by the bear immigration, but probably the reverse. But within the limits of the United States alone, there are now forty-five millions of men where then there were only a few hundred thousand, and yet there is now within that territory much more food per capita for the forty-five millions than there was then for the few hundred thousand. It is not the increase of food that has caused this increase of men; but the increase of men that has brought about the increase of food. There is more food, simply because there are more Men. — Progress & Poverty — Book II, Chapter 3: Population and Subsistence: Inferences from Analogy
read the corresponding passage in Drake's abridgement ...
Humans, however, can extend the conditions that normally limit those species giving our sustenance. (In some cases, our mere appearance will accomplish this.) The reproductive forces of these species then begin to work in service of humans. This increase continues at a pace that our own powers of increase cannot rival. If we shoot hawks, birds will increase; if we trap foxes, rabbits will multiply.
This distinction between humans and all other forms of life destroys the analogy. Of all living things, only humans can manipulate reproductive forces stronger than their own to supply themselves with food. Bird, insect, beast, and fish take only what they find. They increase at the expense of their food. But the increase of humans will increase their food. The population of the United States, once small, is now forty-five million. Yet there is much more food per capita.
It is not the increase of food that has caused the increase of humans -- rather, the increase of humans has brought about an increase of food. There is more food simply because there are more people. This is the difference: Both humans and hawks eat chickens -- but the more hawks, the fewer chickens; while the more humans, the more chickens.