Charles Darwin is notable for developing the theory of evolution. He believed that all living creatures descended from common ancestors, and natural selection was the chief factor. In 1798, Thomas Malthus wrote an Essay on the Principle of Population. He argued, when over population exist in humans and resources remain stable, then population will naturally decrease, due to the lack of resources. Darwin read Malthus' essay and later expanded on it. He included all organisms in his theory, instead of just humans as Malthus did. Darwin knew that sexual reproduction increased variation, but did not know why. Malthus' essay provided the reason Darwin needed to explain his theory. Not all off-spring that is born survives. When over population is met with decreasing resources, then only organisms with favorable circumstances survive. I believe Darwin could have developed his theory of evolution, without the positive influence of Thomas Malthus. Malthus influenced Charles Darwin, as well as Alfred Russel Wallace. Both men independently developed the theory of natural selection, after reading Malthus' essay. The church was a major influence during these times. It taught that God created all life on earth, and that species could not change. Darwin believed that the genetic structure of a population could change, but was hesitant to publish his theory. Anyone that held a belief different of the church, was accused of heresy, and was often jailed or put to death.
http://www.ucmp.berkeley.edu/history/malthus.html