Darwin wrote, “if too many of these parasites grow on the same tree, it perishes and dies.” In context, he was writing about mistletoe growing on apple trees. It was part of his explanation of the struggle for existence in The Origin of Species, Chapter 3.
In the next paragraph he wrote, “Every being … must suffer destruction during some period of its life…, otherwise, on the principle of geometric increase, its numbers would quickly become so inordinately great that no country could support the product.” A few pages later he wrote about what checks the increase.
One thing that checks increase is competition for food among the member of a species and with other species. More significant than that, however, is that we are prey to other species seeking food. Darwin wrote, “very frequently it is not the obtaining food, but serving as prey to other animals” that limits the population of a species. As we struggle for food with members of our own species and with members of others, we are prey to creatures larger than us, and smaller.
Among the most powerful checks on increase is climate, which acts indirectly on population by “favouring other species.” When climate warms, our food supply will shrink. As competition for food among us builds, other species will prosper. In the paragraph after he wrote about climate, Darwin wrote about epidemics that occur “when a species, owing to highly favourable circumstances, increases inordinately in a small tract.” Imagine this Darwinian future: As we struggle with each other for food on the small tract that is earth, small creatures will devour us.
We are like mistletoe overloading a tree. We hope the tree will survive if we switch to fluorescent bulbs and build enough solar panels and windmills. A “sustainable future,” we call it.
It is a ridiculous hope, of course, if Darwin is right. I think he is.
Even if human caused increases in carbon do not adversely change the climate, we are overloading the tree. Our food needs are too great. Too many of us are attached to the same tree, crowding a small green tract, and we continue to increase.