The inexhaustible variety of living things is one of the leading features of the natural world, and one of the chief sources of human inspiration. The study of biological diversity is a scientific discipline in its own right, closely connected to ecology and evolutionary biology. It deals with the issues of how so wide a variety of different kinds of organisms is maintained, how new kinds of organisms come in to existence in evolutionary time, and why organisms become extinct. Unlike many other branches of biology, it is deeply concerned with the different characteristics of particular organisms and therefore emphasizes the detailed study of particular groups. The study of biological diversity is therefore the basis of comparative biology, the development of theories about the natural world through interpreting and predicting the characteristics of organisms living in different circumstances. Our knowledge of diversity is organized through the study of systematics which seeks to understand the history of life and the phylogenetic and genetic relationships of living things. The phylogeny of groups of organisms is an account of its history, a description of its current diversity, and the basis for its classification. An appreciation of diversity and a knowledge of the principles and procedures of systematics are essential in ecology and evolutionary biology and underlie all work in resource utilization and conservation biology.