In the Origin Darwin says that the "struggle for existence" in nature follows from "the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of food, and no prudential restraint from marriage." (Ch. III, First Edition, p.63.) Note though that Darwin is implicitly rejecting "the doctrine of Malthus" with respect to human society. (Note also that he explicitly says that "I use the term Struggle for Existence in a large and metaphorical sense" so that "a plant on the edge of a desert is said to strugglefor life against the drought, though more properly it should be said to be dependent on the moisture." [p.62])
In his Autobiography, Darwin not only notes a parallel of ideas, he says that the mechanism behind natural selection was suggested to him by reading Malthus. However, recent scholarship has revealed that Malthus' influence was not so direct. This is explained in the essay by Gould I cited in an earlier post--"Darwin's Middle Road" in *The Panda's Thumb*.
Whatever the details, though, the main point is that Darwin's theory gives no credence to the idea that Malthus was right about human "overpopulation". More generally, there are many examples in the history of science of mistaken or bad ideas giving rise to good ones. Gould notes that "it is ironic that Adam Smith's [another influence on Darwin] system of laissez faire does not work in his own domain of economics, for it leads to oligopoly and revolution, rather than to order and harmony. Struggle among individuals does, however, seem to the law of nature." An oversimplification, but essentially correct.
I have a brief discussion of these questions in an article on "Marxism and Science" in the latest issue of International Socialism (#79, Summer 1998), which should be available soon at http://www.internationalsocialist.org/pubs/isj.html.
Phil Gasper ptrg at sirius.com 415-522-1895