An ecological point of view is described more fully than the other subjects discussed, so that the student may see the need of familiarity with those tests or criteria by means of which he may be able to determine for himself ecological relations and the validity of ecological studies. The other phases are treated less fully in the discussions and with more detail bibliographically so that this may be a useful source book. The geographical (in the ordinary sense of the word) aspect has been deliberately omitted. The references should be looked upon from the standpoint presented in the general portions of this book, and if the facts and inferences aid in the interpretation of the relations which exist between animals and the sum total of their environments, one may fairly consider that they are of ecological worth. In the arrangement of the references I have tried to group related papers, but many defy any single system. Some of the publications deserve to be in several lists, but little duplication has been made, as this would unduly prolong the lists. The annotations will supplement the titles and their grouping in indicating the contents and importance of the papers for our purpose. It has often been difficult to select from several almost equally valuable and useful papers. Others with different interests, aims, and experience would doubtless make a different choice. It will therefore be a favor, if those who use this handbook and feel that important papers have been excluded, will communicate this fact to the author. This book is not intended as a treatise on the science of ecology; its aim is primarily educational. This is the justification, if any were necessary, for placing emphasis upon the point of view and the importance of an understanding of explanatory processes and of the methods of scientific investigation. Any adequate treatment of this subject would exceed the space of this volume and it is reserved for future elaboration. At present ecology is a science with its facts out of all proportion to their organization or integration. There is thus an immediate need of integration, and this above all requires a clear conception of the scientific method as a tool, and independent thinking as well. The fact that scientific work progresses more rapidly when consciously pursued than otherwise should serve as ample justification for this emphasis.