This article is a lightly edited extract from my book On the Moor: science, history and nature on a country walk.
In On the Origin of Species, Darwin uses a famous simile to illustrate his concept of a struggle for existence:
Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.
For some reason, Darwin groupie that I unashamedly am, I’ve never been enamoured of his wedge simile. Maybe Darwin wasn’t either: he removed it from subsequent editions of his book.
It so happens, Darwin came up with his wedge simile during one of the great eureka moments in the history of science—although I should, perhaps, point out that modern historians of science tend to pooh-pooh the very idea of eureka moments. This particular eureka moment occurred on 28th September 1838, when, as Darwin explained in his autobiography over four decades later (getting the month wrong, and, it has been suggested, misremembering the sudden nature of the revelation):
In October 1838, that is, fifteen months after I had begun my systematic enquiry, 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.
Darwin’s idea of what constituted an amusing read might seem odd to modern readers, but fortune favours the prepared mind. Thanks to observations he had made during and after his five-year voyage around the world aboard HMS Beagle, Darwin was already convinced, as others had been before him, that species evolve. But he, like the rest of them, lacked a physical mechanism by which evolution might occur. Reading the Reverend Thomas Malthus’s essay on the dangers of human population growth, Darwin realised he now had such a mechanism: a struggle for existence, in which better-adapted individuals stand a better chance of surviving and reproducing. He dubbed this mechanism Natural Selection (in contrast to artificial selection: the selective breeding employed by humans to develop desirable traits in domesticated plants and animals). In the same notebook in which Darwin recorded Malthus’s key points, he also jotted down some initial thoughts of his own, and the wedge simile was born:
28th We ought to be far from wondering of changes in numbers of species, from small changes in nature of locality. Even the energetic language of Decandolle does not convey the warring of the species as inference from Malthus. — increase of brutes must be prevented solely by positive checks, excepting that famine may stop desire. — in nature production does not increase, whilst no check prevail, but the positive check of famine & consequently death.
Population is increase at geometrical ratio in far shorter time than 25 years — yet until the one sentence of Malthus no one clearly perceived the great check amongst men. — there is spring, like food used for other purposes as wheat for making brandy. — Even a few years plenty, makes population in Men increase & an ordinary crop causes a dearth. take Europe on an average every species must have same number killed year with year by hawks, by cold &c. — even one species of hawk decreasing in number must affect instantaneously all the rest. — The final cause of all this wedging, must be to sort out proper structure, & adapt it to changes. — to do that for form, which Malthus shows is the final effect (by means however of volition) of this populousness on the energy of man. One may say there is a force like a hundred thousand wedges trying force every kind of adapted structure into the gaps in the oeconomy of nature, or rather forming gaps by thrusting out weaker ones. —Charles Darwin’s Notebook D [Transmutation of species (7-10.1838)]. CUL-DAR123.
Transcribed by Kees Rookmaaker. Darwin Online