only ceases when the sea-birds of the Pacific renew a note that was dropped on the distant Atlantic coast. One of the most fascinating incidents in the life of the explorer in his lonely camps in the great forests or amid the solemn mountains is the coming of the wave of song in the spring and early summer-time which precedes and accompanies the rising of the sun. * * * * * The fascination of the field of study touched upon in this chapter invites an attempt to present an account of some of the more characteristic birds of North America, and to endeavour to convey to the reader some idea of the varied reptilian, fish, and invertebrate life of the continent, but the limitations of space prohibit such a review. Even the great problems dealing with the intimate relation that exists between geographical conditions and the distribution of animals cannot be given more attention than the suggestions already offered. LITERATURE ALLEN, J. A. _The Geographical Distribution of Animals_, in Bulletin of the United States Geological and Geographical Survey of the Territories, vol. iv, Washington, 1878, pp. 313-377; _The Geographical Distribution of North American Mammals_, in Bulletin of the American Museum of Natural History, vol. iv, New York, 1892, pp. 199-243. HEILPRIN, ANGELO. _The Geographical and Geological Distribution of Animals_. International Scientific Series, London and New York, 1897. OSBORN, H. F. _The Rise of the Mammalia in North America._ New York, 1893. COPE, E. D. _Geographical Distribution._ In the Annual Report of the Smithsonian Institution for the year ending June 30, 1898, Washington, 1900, pp. 1199-1234. MERRIAM, C. HART. _Life-Zones and Crop-Zones of the United States_, United States Department of Agriculture, Division of Biological Survey, Bulletin No. 10, Washington, 1898; _Results of a Biological Survey of the San Francisco Mountain Region and Desert of the Little Colorado, Arizona_, United States Department of Agriculture, Division of Ornithology and Mammalogy [Bulletin] No. 3, Washington, 1890; _The Geographical Distribution of Life in North America_, in Proceedings of the Biological Society of Washington, vol. vii, 1892, pp. 1-64. CHAPTER VI GEOLOGY _Introduction_