There has always been a close connection between evolution theory and economics. Both Darwin and Wallace acknowledged their debt to Malthus, whose famous observation that populations have the potential to increase faster than the resources on which they depend became one of the axioms from which the theory of natural selection is derived. The influence of economics did not stop there, of course, but continues to the present day. Terms such as cost-benefit analysis and investment are in common use in evolution, and with their usual meanings. One evolutionist has even described his own book as a "cross between the Kama Sutra and the Wealth of Nations" (Ghiselin 1974, p. 1). Conversely, many economists, for example Marshall, Veblen and, more recently, Friedman (1953), have looked to Darwinism for ideas.