The influence of Darwin's reading of Thomas Malthus's Essay on the Principle of Population is evident in the notebooks that Darwin began writing shortly after he returned from his voyage on the Beagle in 1836. By the spring of 1837, Darwin had embraced the idea of the transmutation of species, and he came close to formulating the logic of natural selection (Notebooks, 193, 258). Then, as he began reading Malthus on September 28, 1838, he grasped for the first time the three steps in the process of natural selection, which he associated with "my Malthusian views" (Notebooks, 375, 397-98, 412-13, 429, 436). First, traits must be passed down by inheritance from parents to offspring. Second, those inherited traits must be variable. Third, there must be a struggle for life so that some inherited variations are favored over others.
In his Autobiography, Darwin described the influence of reading Malthus in the fall of 1838.
"I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here, then, I had at last got a theory by which to work." (120)But while this shows the influence of Malthus's book on Darwin's theory of the organic evolution of species, which was published in his Origin of Species in 1859, we need to see the influence of another book on Darwin's theory of the moral evolution of human beings, which was published in his Descent of Man in 1871. That other book was James Mackintosh's The Progress of Ethical Philosophy.
Mackintosh (1765-1832) was born in Scotland and moved to England in 1787, where he became a prominent Whig member of Parliament, a lawyer, a moral philosopher, and a historian of England. In his Autobiography, Darwin remembered that as a young man of 18, he had met Mackintosh and found him to be "the best converser I ever listened to." Later, Darwin had heard that Mackintosh had told someone,"There is something in that young man that interests me." Darwin was pleased by this, and he remembered "that I listened with much interest to everything which he said, for I was as ignorant as a pig about his subjects of history, politics, and moral philosophy" (55).
Mackintosh's book on ethical philosophy was published in 1837, just when Darwin was beginning to formulate his theory of evolution, and Darwin read the book avidly, presumably because he wanted to extend his theory to "history, politics, and moral philosophy." Darwin marked up his copy of Mackintosh's book and wrote up extensive notes on it (Notebooks, 409, 537, 557-56, 563-64, 587-89, 618-29). Darwin was most interested in Mackintosh's presentation of the moral philosophy of David Hume, Adam Smith, and other writers of the Scottish Enlightenment. These ideas from the Scottish moral philosophers shaped the account of the moral sense that Darwin would later publish in The Descent of Man in 1871.
Perhaps the most notable feature of Darwin's notes on moral philosophy from Mackintosh's book is that God is never mentioned and human morality is understood as a product of natural human experience rather than divine moral design.
I suggest that Malthus was to Darwin's theory of organic evolution as Mackintosh was to his theory of moral evolution. In both cases, Darwin was looking for a theory of spontaneous evolution as opposed to a theory of special creation by divine design. In both cases, God might be seen as the creator of the laws of nature that made both organic evolution and moral evolution possible, but there was no need for special divine intervention to explain the origin of species or the origin of human morality.
Darwin was reluctant to publish his evolutionary theory of the origin of species--he waited 20 years. But he was even more reluctant to publish his evolutionary theory of the origin of human morality--he waited over 30 years!
Darwin did indicate that religious beliefs could be important influences on moral evolution. But he also indicated that moral order could stand on its purely natural grounds even without any religious belief in God as the moral lawgiver.
The possibility of a purely natural morality without religious belief is most clearly stated in Darwin's Autobiography in the section on his religious beliefs. Darwin explains that while he began in his early life as an orthodox Christian, he gradually turned in the latter half of his life toward "scepticism or rationalism." He admits that "the mystery of the beginning of all things is insoluble by us," and this leaves an opening for God as the Creator. But, still, moral life does not require any religious beliefs:
"A man who has no assured and ever present belief in the existence of a personal God or of a future existence with retribution and reward, can have for his rule of life, as far as I can see, only to follow those impulses and instincts which are the strongest or which seem to him the best ones. A dog acts in this manner, but he does so blindly. A man, on the other hand, looks forward and backwards, and compares his various feelings, desires and recollections. He then finds, in accordance with the verdict of the wisest men that the highest satisfaction is derived from following certain impulses, namely the social instincts. If he acts for the good of others, he will receive the approbation of his fellow men and gain the love of those with whom he lives; and this latter gain undoubtedly is the highest pleasure on this earth. By degrees it will become intolerable to him to obey his sensuous passions rather than his higher impulses, which when rendered habitual may be almost called instincts. His reason may occasionally tell him to act in opposition to the opinion of others, whose approbation he will then not receive; but he will still have the solid satisfaction of knowing that he has followed his innermost guide or conscience.--As for myself I believe that I have acted rightly in steadily following and devoting my life to science." (94-95)That moral order can arise as a largely unintended social order based on natural human experience without divine design is important for Darwin's liberalism, because it means that there is no need to coercively enforce religious beliefs as necessary for social order, which makes possible a liberal pluralist society, in which people having conflicting religious beliefs or even no religious beliefs can live together in peace.