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Darwin Correspondence Project

How old is the earth?


A correct globe with the new discoveries, ca 1775
A correct globe with the new discoveries, ca 1775
Cambridge University Library

One of Darwin’s chief difficulties in making converts to his views, was convincing a sceptical public, and some equally sceptical physicists, that there had been enough time since the advent of life on earth for the slow process of natural selection to have produced the plants and animals they saw around them.

Darwin thought of himself as more of a geologist than a zoologist or botanist, especially in his early years, and followed Charles Lyell in pointing to the slow pace of processes such as erosion and deposition as evidence of ‘incomprehensibly vast’ periods of geological time (Origin p. 282).  Unlike his predecessors Darwin risked some calculations.  He concluded that at least 300 million years had elapsed since ‘the latter part of the Secondary period’, that is, the Cretaceous. This was based on the assumed rate at which the Weald, a large area of southern England near his own home in Kent, had been eroded: matching layers of rock in the exposed faces of the widely separated hills of the North and South Downs are the remains of an original dome-shaped feature, the top of which has been worn away.

In the face of a critical review of Origin (Saturday Review, Dec. 24th, 1859), Darwin was forced to back-pedal. He reduced his estimate from at least 300 million years in the first edition of Origin (pp. 285-7), to 150 million in the second, and diluted it still further in later editions.  He included 'Weald Denudation made milder' in a list summarising changes to the second English edition, and although he didn't alter the passage again in the US edition which came out shortly afterwards, he did add an explanatory note:

I confess that an able and justly severe article . . .  shows that I have been rash. I have not sufficiently allowed for the softness of the strata underlying the chalk . . .  Nor have I allowed for the denudation going on on both sides of the ancient Weald-Bay . . .  It has long been my habit to observe the shape and state of surface of the fragments at the bases of lofty retreating cliffs, and I can find no words too strong to express my conviction of the extreme slowness with which they are worn away and removed. I beg the reader to observe that I have expressly stated that we cannot know at what rate the sea wears away a line of cliff: I assumed the one inch per century in order to gain some crude idea of the lapse of years; but I always supposed that the reader would double or quadruple or increase in any proportion which seemed to him fair the probable rate of denudation per century. But I own that I have been rash and unguarded in the calculation.

(Origin US ed., p. 252.)

Darwin’s estimate came under attack as collateral damage in a much wider dispute about the age of the earth between geologists and physicists, most notable among whom was William Thomson, later Lord Kelvin. The stand-off was the subject of an early three-way conversation with Joseph Hooker and Charles Lyell.  ‘I cannot think how you can attach so much weight to the physicists,’ Darwin exclaimed to Hooker, ‘I will maintain to the death that yr case of Fernando Po & Abyssinia is worth ten times more than the belief of a dozen physicists'.  The dispute rumbled on for so long that it was Darwin’s mathematician son George, only a child when Origin was published, who finally gave his father some hope that the physicists would be defeated.  Along the way there was continued support from geologists like Joseph Beete Jukes who objected strongly when Darwin proposed omitting the 'Weald' argument altogether — leading Darwin to exclaim 'How hard it is to please everyone!' — and both Thomas Henry Huxley and Alfred Russel Wallace weighed in at various times.

In 1863, when Origin was in its third edition, Thomson calculated from its assumed rate of cooling that the earth itself was only between 100 to 200 million years old, and he continued to revise this figure inexorably down in succeeding years. By the time Darwin was working on the fifth edition, Thomson had concluded that 100 million years was the upper, rather than lower, limit of the age of the earth. Darwin explained his dilemma to the Scottish geologist, James Croll: ‘I am greatly troubled at the short duration of the world according to Sir W. Thompson, for I require for my theoretical views a very long period before the Cambrian formation.’  The strata of the Cambrian geological era were rich in fossils of a wide range of animals but none had then been found in older layers – a phenomenon later referred to as the ‘Cambrian explosion’ – so that complex life seemed to have sprung suddenly into existence. Evidence of soft-bodied larger animals and of abundant microscopic life in earlier periods has since been discovered, but Darwin was forced to postulate its existence, and explain the lack of evidence by the incompleteness of the fossil record. Even then, there was no getting away from the fact that natural selection could only have produced such a wide variety of Cambrian life over a very long preceding timespan.  

Although Croll responded thoughtfully, suggesting that the earth’s crust could have formed more rapidly on cooling than Thomson allowed, thus pushing back in time the point at which it could first have sustained organic life, this still wasn’t enough for Darwin who, in the meantime, had mentioned the problem to George Darwin, a newly elected fellow of Trinity College, Cambridge: ‘I daresay I shall want much advice about Croll & Thompson & be hanged to them’.

By 1877, George Darwin was working on the effect of the moon’s gravity on the earth, and suggested that Thomson, who had just backed him for fellowship of the Royal Society, should have factored that in to his calculations.   Some anxious months followed, but finally Thomson was convinced by George’s proposal that the friction of the ‘tides’ created in the structure of the earth by the moon’s pull would generate heat and would have slowed the rate of the earth’s cooling.  ‘I also chuckle greatly about the internal Heat. How this will please the geologists & Evolutionists’ his father crowed, ‘Hurrah for the bowels of the earth & their viscosity & for the moon & for all the Heavenly bodies & for my son George’.

The Cretaceous is now held to have spanned the period from 145 to 65 million years ago. The earth is now estimated to be around 4.5 billion years old.