The theory of evolution by natural selection was put forward in the 1850s independently by two men. One was Charles Darwin; the other was Alfred Russel Wallace. Both men had some scientific background, of course, but at heart both men were naturalists. Darwin had been a medical student at Edinburgh University for two years, before his father who was a wealthy doctor proposed that he might become a clergyman and sent him to Cambridge. Wallace, whose parents were poor and who had left school at 14, had followed courses at Working Men's Institutes in London and Leicester as a surveyor's apprentice and pupil teacher. The fact is that there are two traditions of explanation that march side by side in the ascent of man. One is the analysis of the physical structure of the world. The other is the study of the processes of life: their delicacy, their diversity, the wavering cycles from life to death in the individual and in the species. And these traditions do not come together until the theory of evolution; because until then there is a paradox which cannot be resolved, which cannot be begun, about life. The paradox of the life sciences, which makes them different in kind from physical science, is in the detail of nature everywhere. We see it about us in the birds, the trees, the grass, the snails, in every living thing. It is this, the manifestations of life, its expressions, its forms, are so diverse that they must contain a large element of the accidental. And yet the nature of life is so uniform. that it must be constrained by many necessities. So it is not surprising that biology as we understand it begins with naturalists in the 18th and 19th centuries: observers of the countryside, bird-watchers, clergymen, doctors, gentlemen of leisure in country houses. I am tempted to call them, simply, 'gentlemen in Victorian England', because it cannot be an accident that the theory of evolution is conceived twice by two men living at the same time in the same culture — the culture of Queen Victoria in England.