Was it hayfever?

• T. Rex and the Crater of Doom by Walter Alvarez
  Princeton, 236 pp, £18.95, May 1997, ISBN 0 691 01630 5

After the origins of humanity, the question people most like to ask about the distant past is: what killed the dinosaurs? By the end of the Cretaceous Period, 65 million years ago, they had all gone. Their disappearance has long been recognised as abrupt, at least by the leisurely standards of geological time. Nowadays, their extinction inspires and sells books and movies by the dozen. Yet for many years their disappearance was seen as something of a non-question. Textbooks concluded chapters on dinosaurs with a few desultory speculations about their demise before moving on to describe the subsequent Age of Mammals. This was partly due to lack of evidence, but it was also informed by a firm belief in evolution as a progressive force. With the inevitability of clockwork, the dinosaurs had to make way for the superior evolutionary accomplishments of mammals.