Until ten years ago biologists had almost no information about the function of the pineal body, though they supected that it was not that assigned to it by Descartes. It was often stated to be a mere vestige. We now know approximately what it does, and the sequence of the discovery makes an interesting case of scientific procedure. So far as I can see it does not conform to any clear schedule of method such as are proposed by Popper or Kuhn. Certainly biologists were not trying to disprove Descartes. Nor were they seeking to establish a new paradigm of normal science. They were trying to find out what the pineal does, and they succeeded. Anyone who hopes that the discovery has thrown light on the mind/body problem should read no further. But study of the function of the pineal does help us to understand some aspects of visual function a little better and incidentally illuminates some features of human life history. It has been known for a long time that the pineal organ of some lower vertebrates has the character of an eye. It is not fully known how this median eye is used, but some years ago I was able to show that in lampreys it regulates a daily rhythm of colour change. The animals have contractile colour cells and they go pale at night and darken in daytime, but the cycle stops if the pineal is removed (Young, 1935). Little more was found out on these lines until Stebbins and Eakin in I958 showed that the pineal eye of some lizards helps them to regulate their temperature. Reptiles are usually supposed to be cold-blooded animals but in fact they need warmth and are only active after they have been heated by the sun. They come out on a sunny day and bask until hot enough to run about actively and catch their prey or mate. But they have no way of stopping themselves getting too hot (they have no sweat glands), so at midday they must get into the shade. Stebbins and Eakin studied lizards in the hot Californian desert and found that if they covered the pineal or removed it the animals did not bask properly, nor get into the shade when the day became too hot. This work reminds us that an eye has functions beyond seeing shapes, but does not tell us much about the pineal gland of mammals, which of course has no eye-like structure and anyhow lies in the dark, inside the skull. The first clue to mammalian pineal function came from a different source. The Americans McCord and Allen in 1917 undertook to study the effects of extracts of the pineal gland. They found that tadpoles turned pale when given small amounts of extracts of bovine pineal glands and they