This volume can be read at many levels. Not least it is an extraordinary inventory - an illustrated summary of all the Earthly creatures that have ever lived. Whatever living thing the reader comes across, from E coli to an oak tree or an elephant, the book aims to show what kind of creature it is, and how it relates to all the others. Yet there are far too many creatures to present merely as a catalogue. The list of species already described is vast enough - nearly two million - but there could in reality be as many as 30 million different animals, plants, fungi and protists - and perhaps another 400 million different bacteria and archaes. In the 4000 million years or so since life first began on Earth, there could have been several thousand billion different species. The only way to keep track of so many is to classify - placing similar creatures into categories, which nest within larger categories, and so on. As the centuries have passed, so it has become clear that the different groups are far more diverse than had ever been appreciated. Thus Linneus in the 18th century placed all living things in just two kingdoms, Animals and Plants.