As we mentioned in Section 1.1. there can be little doubt that ecology was responsible for the development of the theory of diversity and that it is still the main field of application. Throughout this chapter, the classes C1,C2,…,C S refer to the various species of some taxonomic group of animals or plants. The possibility of applying the theory to higher order classification, for example by viewing the elements as species and the classes as genera, will be discussed in Chapter 7.