J. D. Hudson writes: Anybody reading the recent paper on the Cretaceous-Tertiary extinctions in this Journal (MacLeod et al. 1997) should also read the papers in Geological Society of America Special Paper 307 (Ryder et al. 1996), including especially the commentaries by Ryder and by Glen. These authors summarize the background to the debate over the nature and timing of events around the boundary, and they point to some of the biases that have affected almost all contributors to it. I believe that MacLeod et al. have got it wrong in denying the reality of an abrupt and impact-related catastrophe at the K–T boundary as the major cause of the extinctions at that time, and that their presentation reveals a gradualistic bias in its choice of quotations. But first, in order to acknowledge the points made by Glen, in his exposé of the tendency we all have to prefer corroborative to inconvenient evidence, I should make my own prior standpoint clear. I have written nothing about the K–T boundary previously apart from a brief mention in a review of palaeo-atmospheres (Hudson 1989). I have given many lectures about it, mainly to undergraduate classes. I was long ago intrigued by the attempts by Bramlette (1965), Tappan (1968) and Gartner & Keany (1978) to explain large-scale turnovers in evolutionary history in terms of world-wide events affecting the ocean–atmosphere system, surely one of the most important endeavours historical geologists can undertake. In the post-Alvarez debate, I have been impressed

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