In 1869, 10 years after the publication of On the Origin of Species, Darwin wrote to his close friend, the great botanist Joseph Hooker, as follows: "If I lived twenty more years and was able to work, how I should have to modify the Origin, and how much the views on all points will have to be modified! Well, it is a beginning, and that is something".

Darwin lived 13 more years after writing this letter, and he did manage to modify the theory of evolution by natural selection, by expanding it in The Descent of Man in 1871 to include human origins, and in The Expression of the Emotions in Man and Animals in 1872 to address the evolution of instinct. And thus, if we add his first book, Voyage of the Beagle, was complete what can be fairly called the four great books of Darwin, although he authored many other influential articles and books during his career.

Great scientific discoveries are like sunrises. They illuminate first the steeples of the unknown, then its dark hollows. The four major books have spread light not just on the living world but, fundamentally, the human condition. They have not lost their freshness: more than any other work in history's scientific canon, they are both timeless and persistently inspirational.

The 130 years ensuing after the completion of the four great books has seen an enormous growth of the Darwinian heritage. Joined with molecular and cellular molecular biology, that accumulated knowledge is today the substance of modern biology. Its centrality justifies the famous remark made by the evolutionary geneticist Theodosius Dobzhansky, the foremost of the modern synthesizers of the 20th century. He famously said in 1973, "nothing in biology makes sense except in the light of evolution."

In fact, nothing in science as a whole has been more firmly established by interwoven factual information, or more illuminating than the universal occurrence of biological evolution. Further, few natural processes have been more convincingly explained than evolution by the theory of natural selection, or as it has been popularly called, Darwinism.

The four books, when read as a set chronologically, flow like a well-wrought narrative, tracing the development of Darwin's thought across almost all of his adult life.

This is an extract from a speech by the author of 'Sociobiology' being broadcast at the Royal Institution in London today