A theory developed by Thomas R. Malthus (1766-1834) and published as An Essay on the Principle of Population in 1798. It can be summarised as follows
1. a plentiful food supply and supporting agricultural resources are necessary for the continuing existence of human populations, however
2. human populations increase at geometrical rates (i.e. 1, 2, 4, 16, 32 etc.), whereas food supply does so at arithmetical rates (i.e. 1, 2, 3, 4, 6 etc.), meaning that population growth will always exceed the resources for subsistence and which in turn leads to
3. famine, disease and war (or Malthusian catastrophes) in which some would survive and others would not, and which required that
4. the effects of the two unequal growth rates must be kept in balance if such catastrophes were to be avoided, however
5. humans tend not to limit population size voluntarily by means of what Malthus called ‚aapreventive checks‚aa, and therefore population reduction tends to be accomplished by the ‚appositive checks‚aa of famine, disease and war. Before reading the Essay, Darwin had thought that living animals and plants reproduced just enough individuals to keep populations stable. After reading the expanded version (1803) in which Malthus brought empirical evidence to bear on his theory derived from his extensive travels in Germany, Russia and Scandinavia, Darwin (like Wallace) immediately saw the implications for his theory of natural selection
The variation he had observed in wild populations would produce some individuals better equipped to thrive and survive in a competitive environment, and that they would tend to leave more offspring than their conspecifics such that over many generations their traits would come to dominate the population. As Darwin wrote later
‚aaThe result of this would be the formation of new species‚aa and ‚aaHere, then, I had at last got a theory by which to work.‚aa Malthus arrived at his theory from discussions he had with his father about the thesis of the ‚aaindefinite perfectibility of society‚aa as propounded at the time by William Godwin (1756-1836) in England and Marie Jean Antoine de Caritat Condorcet (1743-1793) in France, and which contained the claim that high fertility contributed to national wealth. The theory continues to this day to influence debates about population growth and its immediate environmental impact. One consensus of opinion is that the enhancement of women educationally, economically and politically is an important mechanism by which to reduce birth rates to more sustainable levels, and for which there is strong empirical evidence
. See Eugenics, Geology, Darwinism, r- and k-selection theory, Theory of natural selection