The Structure and Distribution of Coral Reefs - Publication of Theory - Structure of Book

Structure of Book

The book has a tightly logical structure, and presents a bold argument. Illustrations are used as an integral part of the argument, with numerous detailed charts and one large world map marked in colour showing all reefs known at that time. A brief introduction sets out the aims of the book.

The first three chapters describe the various types of coral reef, each chapter starting with a section giving a detailed description of the reef Darwin had most information about, which he presents as a typical example of the type. Subsequent sections in each chapter then describe other reefs in comparison with the typical example. In the first chapter, Darwin describes atolls and lagoon islands, taking as his typical example his own detailed findings and the Beagle survey findings on the Keeling Islands. The second chapter similarly describes a typical barrier reef then compares it to others, and the third chapter gives a similar description of what Darwin called fringing or shore reefs. Having described the principal kinds of reef in detail, his finding was that the actual surface of the reef did not differ much. An atoll differs from an encircling barrier reef only in lacking the central island, and a barrier reef differs from a fringing reef only in its distance from the land and in enclosing a lagoon.

The fourth chapter on the distribution and growth of coral reefs examines the conditions in which they flourish, their rate of growth and the depths at which the reef building polyps can live, showing that they can only flourish at a very limited depth. In the fifth chapter he sets out his theory as a unified explanation for the findings of the previous chapters, overcoming the difficulties of treating the various kinds of reef as separate and the problem of reliance on the improbable assumption that underwater mountains just happened to be at the exact depth below sea level, by showing how barrier reefs and then atolls form as the land subsides, and fringing reefs are found along with evidence that the land is being elevated. This chapter ends with a summary of his theory illustrated with two woodcuts each showing two different stages of reef formation in relation to sea level.

In the sixth chapter he examines the geographical distribution of types of reef and its geological implications, using the large coloured map of the world to show vast areas of atolls and barrier reefs where the ocean bed was subsiding with no active volcanos, and vast areas with fringing reefs and volcanic outbursts where the land was rising. This chapter ends with a recapitulation which summarises the findings of each chapter and concludes by describing the global image as "a magnificent and harmonious picture of the movements, which the crust of the earth has within a late period undergone". A large appendix gives a detailed and exhaustive description of all the information he had been able to obtain on the reefs of the world.

This logical structure can be seen as a prototype for the organisation of On the Origin of Species, presenting the detail of various aspects of the problem, then setting out a theory explaining the phenomena, followed by a demonstration of the wider explanatory power of the theory. Unlike the Origin which was hurriedly put together as an abstract of his planned "big book", Coral Reefs is fully supported by citations and material gathered together in the Appendix. Coral Reefs is arguably the first volume of Darwin's huge treatise on his philosophy of nature, like his succeeding works showing how slow gradual change can account for the history of life. In presenting types of reef as an evolutionary series it demonstrated a rigorous methodology for historical sciences, interpreting patterns visible in the present as the results of history. In one passage he presents a particularly Malthusian view of a struggle for survival – "In an old-standing reef, the corals, which are so different in kind on different parts of it, are probably all adapted to the stations they occupy, and hold their places, like other organic beings, by a struggle one with another, and with external nature; hence we may infer that their growth would generally be slow, except under peculiarly favourable circumstances."