The authors have long felt the need for a treatise on the principles and methods of taxonomy. Such a work should be useful not only as an adjunct to teaching but also as a reference work for the practicing taxonomist and as a source of information to the general biologist. An analysis and full statement of the often disputed principles on which the taxonomic method is based are urgently needed. We share the view of O. W. Richards (1947) that "it is less the findings of taxonomy than its principles and methods which need to be taught" and understood. We believe that taxonomy is an important branch of biology which deals not only with the identification and classification of natural populations but with objectives that go well beyond these fundamental activities. [...] In attempting to bring together the more important elements of modern taxonomic theory and practice, we have, of necessity, selected our materials primarily from the point of view of the student of living animals and have chosen illustrative examples with preference from our mvn work. The problems of the paleontologist, microbiologist, and botanist have been taken into consideration as far as practicable, but the materials of these groups are often sufficiently different to require different approaches to the solution of taxonomic problems. Nevertheless, there is much common ground of theory and method shared by the workers in these diverse fields, and it is to be hoped that at some time in the not too distant future all biological taxonomy may be viewed· as a single cohesive field. If this book, by focusing attention on the problems of the systematic zoologist, serves as a step in that direction, one of its goals will have been achieved. If it also assists in stimulating a more critical evaluation of taxonomic theory and methods and in a wider dissemination of knowledge concerning them, the authors will feel that their labors have been justified. [From the Preface]