Anything that evolves in contradiction to the twin engines of random mutation and natural selection is a potential threat to Darwin's theory. John Cairns, a Harvard University geneticist, discovered just such a contradiction in 1988. He found that some of the mutations in the bacterium E coli seemed to be calculated responses to environmental pressures.

On the face of it, it was a blatant heresy -how can an organism sense its needs in a particular environment and then mutate accordingly? Orthodox theory holds that the environment selects mutations, but it cannot direct them. According to Lamarck, the 18th century naturalist who suggested that organisms evolve by acquiring favourable characteristics, the opposite is true: genetic change occurs in response to environmental pressure. This heresy has never mustered much support among modern biologists who have always been resistant to the idea of the environment restricting genetic change to beneficial mutations.

Had Cairns and his colleagues found existence of directed mutagenesis? Studies have strengthened the Harvard observations. Three years ago, a study by Barry Hall, a geneticist at the University of Rochester. US, discovered an hitherto unrecognised talent in E coli -the ability to manipulate the rate at which favourable combinations of chance mutations appear in its genes.