Thomas Malthus (1766 – 1834)
Malthus was an English clergyman and economist who published his theory in 1798 under the title An Essay on the Principle of Population. Malthus argued that human populations grow geometrically while food supply grows arithmetically. Geometric growth means that the increase is proportional to the function’s current value. An example of geometric growth is the series 2, 4, 8, 16, 32, 64, 128, etc. Arithmetic growth means the increase is by a fixed amount. An example of arithmetic growth is the series 2, 4, 6, 8, 10, 12, 14, etc. Since the population grows much faster than the food, there comes a time when the population exceeds the resources. Darwin read Malthus and generalized the theory to all species. He realized that even species that reproduce slowly, like elephants, can exceed their resources given enough time.
So there must be some competition between individuals who fight for limited resources. Since they are insufficient, there must be a struggle for survival. Furthermore, Darwin knew there is variability among individuals in any species. He concluded that there should be some traits characterizing the better-adapted individuals. Those who are the fittest reach reproductive age and leave offspring who inherit their traits. The unfit die before reproducing, leave no offspring, and their features disappear. That was exactly the way how populations were evolving over time.