Order in Diversity

Zoologists have named more than 1.5 million species of animals, and thousands more are described each year. Some zoologists estimate that the species named so far constitute less than 20% of all living animals and less than 1% of all those that have existed in the past.

Despite its magnitude, the diversity of animals is not without limits. Many conceivable forms do not exist in nature, as our myths of minotaurs and winged horses demonstrate. Characteristic features of humans and cattle never occur together in nature as they do in the mythical minotaur; nor do characteristic wings of birds and bodies of horses occur naturally as they do in the mythical horse, Pegasus. Humans, cattle, birds, and horses are distinct groups of animals, yet they do share some important features, including vertebrae and homeothermy, that separate them from even more dissimilar forms such as insects and flatworms.

All human cultures classify their familiar animals according to various patterns in animal diversity. These classifications have many purposes. Some societies classify animals according to their usefulness or destructiveness to human endeavors; others may group animals according to their roles in mythology. Biologists group animals according to their evolutionary relationships as revealed by ordered patterns in their sharing of homologous features. This classification is called a “natural system” because it reflects relationships that exist among animals in nature, outside the context of human activity. Systematic zoologists have three major goals: to discover all species of animals, to reconstruct their evolutionary relationships, and then to classify them accordingly.