The zoölogical collections described in the fifth volume of Reports, Survey west of the Hundredth Meridian,[P] were all obtained in that zoölogical province known as the "Campestrian region," from the great plains which it includes. There the animal colors are pale and tend toward uniformity, corresponding to the low rainfall of from three to twenty inches per year. In this peculiarity, and also in comparison with the surrounding more humid regions, the district of country in which the Government surveys are now carried on sustains the general theory that coloration in animals is closely dependent on rainfall, a humid atmosphere serving to cloak the sun's rays and preserve the natural dyes (mostly organic) from bleaching out. Dr. Yarrow thinks that the entirely rainless parts of this vast Campestrian region may ultimately deserve recognition as a separate zoölogical province. The observations made as to the mimicry of color which some animals, especially reptiles, exert or suffer lead him to believe that "a law may yet be formulated in this respect which will equally apply to all classes of animals." This mimicry was especially noticed in serpents and lizards found near red sandstone deposits, the well-known little _Phrynosoma_, or horned toad, being greenish gray, nearly white, or deep red, as it was found on the plain, the alkali flat, or the sandstone soil. But however profound the change, the skin returned to its normal color within a day or two after removal from the determining locality. In regard to the rattlesnake, we have the welcome information that it is apparently decreasing in numbers, and the less agreeable fact that with other serpents, it principally frequents the neighborhood of settlements. The collections of all kinds made by the explorers prove to be unexpectedly perfect in spite of the rapidity with which they are forced to move, and losses by fire and railroad accident. The report upon these collections is drawn up with the care and thoroughness that are such creditable features of recent American official work. A copious bibliography and synonomy is attached to the descriptions of species. The allotment of reports is as follows: Geographical Distribution, Dr. H. C. Yarrow; Mammals, Dr. Elliott Coues and Dr. Yarrow; Birds, H. W. Henshaw; Batrachians and Reptiles, Dr. Yarrow; Fishes, Prof. E. D. Cope and Dr. Yarrow; Insects, E. T. Cresson, E. Norton, T. L. Mead, R. H. Stretch, C. R. Osten-Sacken, H. Ulke, R. P. Uhler, Cyrus Thomas, H. A. Hagen; Mollusca, Dr. Yarrow. These names show how carefully the head of the survey, Lieutenant Wheeler, has sought assistance in the important work of classification. But these are by no means all from whom he and his assistants acknowledge service. The list given in the preface numbers more than forty persons, and includes the best known specialists in this country. Forty-five plates, colored when necessary, accompany the text. In every