Shortly after Darwin went to Down he began one long and memorable experiment, which in itself casts a flood of light upon his patient and painstaking method of inquiry. Two years before, he had read at the Geological Society a paper on the 'Formation of Mould,' which more than thirty years later he expanded into his famous treatise on the 'Action of Earthworms.' His uncle and father-in-law, Josiah Wedgwood, suggested to him that the apparent sinking of stones on the surface might really be due to earthworm castings. So, as soon as he had some land of his own to experiment upon, he began, in 1842, to spread broken chalk over a field at Down, in which, twenty-nine years later, in 1871, a trench was dug to test the results. What other naturalist ever waited so long and so patiently to discover the upshot of a single experiment? Is it wonderful that a man who worked like that should succeed, not by faith but by logical power, in removing mountains?
Unfortunately, we do not know the exact date when Darwin first read Malthus. But that the perusal of that remarkable book formed a crisis and turning-point in his mental development we know from his own distinct statement in a letter to Haeckel, prefixed to the brilliant German evolutionist's 'History of Creation.' 'It seemed to me probable,' says Darwin, speaking of his own early development, 'that allied species were descended from a common ancestor. But during several years I could not conceive how each form could have been modified so as to become admirably adapted to its place in nature. I began therefore to study domesticated animals and cultivated plants, and after a time perceived that man's power of selecting and breeding from certain individuals was the most powerful of all means in the production of new races. Having attended to the habits of animals and their relations to the surrounding conditions, I was able to realise the severe struggle for existence to which all organisms are subjected; and my geological observations had allowed me to appreciate to a certain extent the duration of past geological periods. With my mind thus prepared I fortunately happened to read Malthus's "Essay on Population" and the idea of natural selection through the struggle for existence at once occurred to me. Of all the subordinate points in the theory, the last which I understood was the cause of the tendency in the descendants from a common progenitor to diverge in character.'