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Stephen Jay Gould has a reputation for writing works which claim to challenge accepted views on the pattern and process of evolution. This was particularly true of his theory of punctuated equilibria which held that evolutionary lineages are characterised by long periods of morphological stability punctuated by much shorter periods of rapid change. Whether this pattern of change, if indeed an accurate analysis of the fossil record, is really as fundamental and novel a concept in evolution as claimed, has been a topic of some controversy. Possibly the phenomenon simply reflects the fact that, while evolution proceeds by the accrual of small genetic changes, some of the changes can have major morphological effects. This may be particularly true of genes which control developmental switches. Similar criticisms could be levelled at this book. It too claims to challenge the established view that evolution has been a process of gradually increasing complexity in the organisation of living creatures. The focus of this challenge is a description of the fossilised remains of organisms which lived in early Cambrian times and which have been exceptionally well preserved in deposits known as the Burgess Shale in the Canadian Rockies. Originally discovered early this century, reappraisal of the Burgess specimens over the last twenty years has revealed a range of body plans far wider than that found among living organisms today. Among 28 species categorised, eight belong to previously unknown phyla and thirteen to previously unknown classes of arthropod. This initial flowering of variety early in the history of animal life was quickly followed by the extinction of most body plans, all subsequent evolution being variations on the few surviving themes. Thus the much larger number of species living today is based on a vastly smaller number of body plans than could be found in the early Cambrian seas. This is certainly an interesting feature of the evolutionary process, but it is arguable whether its assimilation requires the radical revision of previously held views which Gould suggests. Gould considers the implications of this decimation of phyla and argues that the criteria of success were not superior adaptation but pure chance. Accordingly the history of life could have taken a quite different course and the probability that any of the other permutations would have led to the emergence of beings with the power of intellectual thought is vanishingly small. Mankind should therefore see his own intellectual capacity not as the inevitable culmination of an evolutionary process which generates ever-increasing complexity but as a most improbable outcome of chance events. The book is a good read, although Gould (an American) is clearly determined to avoid the risk of understatement which he characterises as "the British norm". The story of the discovery of the Burgess Shale and the tortuous path to its correct interpretation is a salutary lesson in the dangerous role of expectation in the conduct of scientific research. The descriptions of the many exotic creatures which populated early Cambrian seas are a delight and the illustrations painstakingly detailed. Even the complexities of arthropod anatomy and classification are presented in a clearly comprehensible manner. Every argument in this book rests on the underlying assumption that standard taxonomic classifications have evolutionary significance, yet this assumption is scarcely made explicit except for the brief assertion that "taxonomy is … the expression of evolutionary arrangement" and so never justified. Certainly the criteria for distinguishing classes of arthropod look as if they ought to have fundamental evolutionary significance but, in view of the controversy which surrounds such matters, some analysis would have been welcome. In particular, consideration of the relationship between these criteria and potential developmental pathways is an area worthy of full exploration which is sadly ignored. ROBERT A. PEEL | ||||||