Many works on the subject of color have been published, but most of them are purely technical, and pertain to the physics of color, the painter's needs, or to some particular art or industry alone, or in other ways are unsuited for the use of the zoologist, the botanist, the pathologist, or the mineralogist; and the comparatively few works on color intended specially for naturalists have all failed to meet the requirements, either because of an insufficient number of color samples, lack of names or other means of easy identification or designation, or faulty selection and classification of the colors chosen for illustration. More than twenty years ago the author of the present work attempted to supply the deficiency by the publication of a book[1] containing 186 samples of named colors, but the effort was successful only to the extent that it was an improvement on its predecessors; and, although still the standard of color nomenclature among zoologists and many other naturalists, it nevertheless is seriously defective in the altogether inadequate number of colors represented, and in their unscientific arrangement. Fully realizing his failure, the author, some two or three years later, began to devise plans, gather materials, and acquire special knowledge of the subject, in the hope that he might some day be able to prepare a new work which would fully meet the needs of all who have use for it. Unfortunately, his time has been so fully occupied with other matters that progress has necessarily been slow; but after more than twenty years of sporadic effort it has at last been completed. Acknowledgments are due to so many friends for helpful suggestions that it is hardly possible to name them all, or to specify the extent or kind of help which each has rendered; but special mention should be made of Mr. LEWIS E. JEWELL, of Johns Hopkins University; Dr. R. M. STRONG, of the University of Chicago; Prof. W. J. SPILLMAN, of the U. S. Department of Agriculture; Mr. WILLIAMS WELCH, of the U. S. Signal Service; Mr. MILTON BRADLEY, of Springfield, Mass.; Dr. P. G. NUTTING, of the U. S. Bureau of Standards; Mr. P. L. RICKER, of the Bureau of Plant Industry, U. S. Department of Agriculture; and Mr. J. L. RIDGWAY, of the U. S. Geological Survey. The late Professor S. P. LANGLEY, then Secretary of the Smithsonian Institution, was good enough to take a kindly interest in this undertaking and gave the author assistance for which he is glad to make acknowledgment. More than to all others, however, is the author deeply indebted to Mr. John E. THAYER, of Lancaster, Mass., and Señor Don JOSÉ C. ZELEDÓN, of San José, Costa Rica, for aid so indispensible that without it the work could not have been completed.