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What is a wilderness? This is the question tackled in a masterly introduction to Wilderness: Earth's Last Wild Places that is signed by the book's 26 contributing authors and photographers.

It explores the Germanic origin of the word ("wild-beast-home"), how other languages treat the concept and different modern approaches to defining wildernesses. Norman Myers' "hotspots" principle is explained, even though it is now rather out of fashion, as is Russell Mittermeier's strange "megadiversity country" approach, which highlights "the 17 countries that occupy a very special position in global conservation" and that was the subject of an earlier companion volume.

Of the many definitions, the one preferred by the authors as the most "lyrical" comes, surprisingly, from the US Wilderness Act of 1964. This calls a wilderness "an area where the Earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain, Iretaining its primeval characterI affected primarily by the forces of nature, with the imprint of man's work substantially unnoticeable; has outstanding opportunities for solitude or a primitive and unconfined type of recreationI and may also contain ecological, geological, or other features of scientific, educational, scenic, or historical value".

The introduction reminds us of the importance of some wildernesses, notably, their unsurpassed biodiversity, the role of forests as carbon sinks and the fact that the Antarctic ice contains an amazing 70 per cent of the world's fresh water. There is then a discussion of protective legislation or lack of it, and the roles of non-governmental organisations - notably, the International Union for Conservation of Nature, for which lead author Mittermeier chairs the primate specialist group, and Conservation International, the fast-growing charity of which Mittermeier is president and that was the main promoter of this volume. (One criticism of Wilderness is its frequent plugs for what CI has achieved in a given habitat.)

The criteria used in choosing wilderness areas are explained: size (it must be big enough), "intactness" and "biodiversity". There is also an interesting concept of "human population density", with the chosen criterion being five or fewer inhabitants per square kilometre, after urban dwellers have been subtracted. This means that some wilderness areas in this book contain gigantic cities and many millions of citizens - notably, the Sahara/Sahel (26 million city folk subtracted before calculating average population), the Caatinga of central Brazil (18 million), boreal forests (15 million), the coastal deserts of Peru and Chile (12 million) and so forth. The authors argue that such areas contain wilderness beyond their sprawling metropolises and shanty towns.

Having established their parameters, the authors proceed to ignore many of them. Their chosen wildernesses range from gigantic to trivial. There is a short chapter on the entire continent of Antarctica (almost 14 million km2), others on "boreal forests" and "Arctic tundra" covering much of Russia, Canada and Alaska (some 16 million km2 and 9 million km2 respectively), and another on "Sahara/Sahel" (11 million km2). By contrast, we have chapters of comparable length on the Sundarbans, the coastal wetlands between Bengal and Bangladesh occupying a mere 10,000 km2, and another on the Tasmanian World Heritage Wilderness Area of under 14,000 km2. "European Mountains" get a chapter (in which the Alps, Carpathians and Scottish Highlands are juxtaposed), whereas the Himalayas-Karakoram, Andes and southern Rockies are omitted. Apart from the Sundarbans, no South Asian wilderness is covered; and for Southeast Asia we have a chapter on the entire island of New Guinea, but nothing on Borneo or any other part of Indonesia. While the Sahara/Sahel, Asian and Arabian deserts are each handled in a single chapter, the smaller deserts between the US and Mexico have four entries (Mohave, Sonoran and Baja California's, Greater Chihuahan, and Colorado Plateau). Faced with such eccentricities you suspect that the main criteria for inclusion are that the authors should have expert knowledge, beautiful photographs or that CI should be active there.

Wilderness is particularly strong on South America, with an excellent 50-page chapter on Amazonia, others on the Chaco, Caatinga (the dry woods and scrublands of central and north-eastern Brazil), Pantanal, Llanos, Patagonia, Pacific coastal deserts and (quirkily) the Banados del Este in coastal Uruguay. The texts are very good: informative, up to date and thorough summaries of each habitat and ecosystem. Each starts with biodiversity, highlighting endemic and the most glamorous flagship species, then a few words on "human cultures", followed by a discussion of "threats" and "conservation" efforts.

I particularly liked the chapter on the Pantanal, the world's largest contiguous wetland (located between Brazil, Bolivia and Paraguay). It has plenty of accurate information and thoughtful appraisals of what is going right and wrong in this magnificent place. As usual in South America, habitat destruction to create cattle pasture or for logging is a major concern, but Mittermeier (and the others who contributed to this chapter) rightly stress overfishing and the introduction of alien grasses, fish species and feral pigs as greater threats.

On a positive note, ecotourism is seen as a money-spinner that has led farmers to restrain hunting of flagship species - the jaguar (of which the Pantanal variety is the most abundant and heaviest at up to 200kg), tapir, giant otter and Paraguayan cayman, all of which are increasing in numbers.

The writing in Wilderness is quite scholarly, with no flights of literary fantasy. Most chapters would form admirable handbooks for someone lucky enough to visit a wilderness. The fantasy in this book is left to the hundreds of beautiful colour illustrations. Alongside breathtaking views and exuberant vegetation, we have many exotic animals, insects and handsome indigenous peoples - no scruffy slum-dwellers here, only tribals in their body paint and finery.

Many photographs are by the outstanding Mexican wildlife photographer Patricio Robles Gil, who is credited on the title-page (but not the cover) as the book's editor. Gil used his position as an editor to do full justice to the ravishing pictures. They are superbly printed and in gigantic format.

This is my main criticism of this book: it is far too large for modern bookshelves. Measuring a whopping 36cm by 29cm, it is not just a doorstop but could serve as a doorstep. Another criticism is the dichotomy between the pictures, selected for their beauty, and the factual text; also the lack of an index.

Perhaps CI and the book's sponsor, cement company Cemex, could be persuaded to issue some of the chapters as separate booklets that could be taken into the field or used in course work. The pictures would lose some of their majesty, but it would be more helpful than this 6kg titan (almost a stone in weight), which cannot fit onto a photocopier and threatens to break all but the most robust of coffee tables.

John Hemming was director, Royal Geographical Society, from 1975-96, and has been on expeditions in or visited many wilderness areas.