In studying the activities of the individual animal, the normal environment to which it is _attuned_ forms the natural unit or basis for study. All processes which modify or change such an optimum will stimulate the animal, cause responses and adjustments. There are many degrees or stages in the development of these optima which change with the functional rhythms and with the development of the animal. There are those conditions which influence the activity of certain functions or organs; those which influence the general vital processes in general, the vital optimum; those which appear to condition the best development of certain families, genera, etc.; and finally those of animal associations. Of course these grade imperceptibly into one another, and a single animal may in its development, traverse all of these stages in the development of its associational optimum. Optima thus have histories, and their development and laws of transformation are of the most fundamental ecological importance (Adams, 1904, 1909; Blackman, 1905, 1906; Shelford, 1911, 1912). In the selection of these papers I have been guided by several considerations. Studies of common animals are given preference, also those papers which by their method of treatment and point of view are especially suggestive and may act as models for further study, and particularly those papers which treat of the activities from the standpoint of their changes, cycles, modifiability, and development. It is to such papers that we must look for suggestions regarding the methods or processes of adjustment between the animal and the environment. All of these papers are not equally ecological, but all will be of much utility in ecological work. (General works are listed first, followed by special papers arranged alphabetically.) _1. General Physiology and Development_ VERWORN, M. 1899. General Physiology. pp. 615. New York. A very good general summary, but somewhat out of date. There is a German edition of 1909 (fifth). ROSENTHAL, J. 1901. Lehrbuch der Allgemeinen Physiologie. pp. 616. Leipzig. FÜRTH, O. VON. 1903. Vergleichende chemische Physiologic der niederen Tiere. pp. 670. Jena. An excellent general work. Indispensable. Full references. PÜTTER, A. 1911. Vergleichende Physiologie. pp. 721. Jena. WINTERSTEIN, H. (Editor). 1910. Handbuch der vergleichenden Physiologie. (To be completed in four volumes.) Jena. HAMMARSTEN, O. (Trans. by J. A. MANDEL.) 1911. A Text-Book of Physiological Chemistry. Sixth Edition, pp. 964. New York. DAVENPORT, C. B. 1908. Experimental Morphology, pp. 509. New York. Excellent summaries and full references to the influence of various stimuli upon growth and upon protoplasm. LOEB, J. 1906. The Dynamics of Living Matter. pp. 233. New York. PRZIBRAM, H. 1910. Experimental-Zoologie. 3. Phylogenese. pp. 315. Leipzig and Vienna. JENNINGS, H. S. 1906. Behavior of the Lower Organisms, pp. 366. New York.