The two main aims of this book are to increase the general availability of classical contributions to animal biology and to present the development of thought in this field in the words of those who produced it.

The first of these aims is realized by assembling in one volume works previously scattered and, in some cases, rather rare.

The second object, that of tracing the principal patterns of development in the field, is made possible through the selection of appropriate materials and the inclusion of brief critical comments indicating the historical position of each work and its author.