According to Darwin, what maintains population size within reasonable limits?

The concept that Darwin proposed about population size being maintained within reasonable limits is closely linked to the idea of environmental carrying capacity, where resources are finite, and populations cannot grow indefinitely. In any given environment, factors such as food availability, space, and predation will constrain how large a population can grow.

Darwin highlighted that organisms tend to produce more offspring than can possibly survive, leading to competition for limited resources. This competition naturally regulates the population size, as not all individuals will survive to adulthood, and not all will reproduce successfully. Thus, population sizes stabilize as a result of these ecological pressures, keeping them within reasonable limits rather than allowing unchecked growth.

Choosing this option reflects an understanding of the balance in ecosystems and the limitations that natural selection imposes on population dynamics, which are fundamental concepts in Darwin's theory of evolution.