vegetation of the continent since the Tertiary period are still in progress. With far-reaching and exceedingly slow changes in climatic conditions and in elevation above the sea due to upheaval and denudation, the plants of our forests, prairies, and mountainsides, are being moved here and there, in ever-changing combinations. Nature thus secures a rotation in the vegetation of a region, as the careful husbandman varies his crops from year to year. The suggestion in this connection furnished by geologists is that we are living in a spring-time following the great winter, known as the Glacial epoch, and that the tropical, temperate, and subarctic forests are migrating northward in an orderly march, and each in turn ascending higher and higher on the more lofty mountains. LITERATURE MUIR, JOHN. _The Mountains of California._ T. Fisher Unwin, London, 1894. PINCHOT, GIFFORD. _A Primer of Forestry._ United States Department of Agriculture, Bulletin No. 24, Washington, D. C., 1899. SARGENT, C. S. _Report on the Forests of North America._ Tenth Census of the United States, vol. ix, Washington, D. C., 1884. United States Geological Survey. Reports on Forestry. CHAPTER V ANIMAL LIFE A common ground to zoologists and geographers in the exploration of which they derive mutual pleasure from assisting each other, is the geographical distribution of animals. In this connection the fauna of North America presents perhaps even more interesting problems than does its flora. GENERAL PRINCIPLES OF GEOGRAPHICAL DISTRIBUTION In the study of the distribution of animals over a continent, the discovery of the laws determining the intangible boundaries which the members of a species may not pass is even more difficult than the similar task in the case of plants. Plant species for the most part advance and retreat slowly as conditions change, and, with minor exceptions, there is no freedom of movement for the individual; but animals, and especially the higher forms, are sensitive to even slight changes in their environment, and there is more or less individual freedom to travel over the land, to fly through the air, or to swim through the water. Why the members of a given species which have apparently unlimited power to travel should be confined to a certain and frequently a narrowly circumscribed area has excited the curiosity of man for many centuries.