The number of these large animals living between the Mississippi and Rocky Mountains previous to 1872, even after being hunted for centuries by the Indians and killed in constantly increasing numbers each year by white men for half a century, can only be reckoned in millions. Many trustworthy observations are on record, however, from which the vastness of the herds can be approximately judged. For example, R. I. Dodge thus refers to a herd which he passed through in the Arkansas Valley in May, 1871, when it was moving northward: "The great herd could not have averaged, _at rest_, over 15 or 20 individuals to the acre, but was not less than 25 miles wide, and from reports of hunters and others it was about five days in passing a given point, or not less than 50 miles deep. From the top of Pawnee Rock I could see from 6 to 10 miles in almost every direction. This whole vast space was covered with buffalo, looking at a distance like one compact mass, the visual angle not permitting the ground to be seen." From this and other observations, W. T. Hornaday has estimated that the herd referred to numbered not less than 4,000,000 individuals. From the records kept by railroad companies of the number of skins shipped over their lines and other evidences, it has been computed that of the southern herd mentioned above over 3,500,000 individuals were killed during the years 1872, 1873, and 1874. The northern herd seems to have been at least as large as the southern one, so that the entire number on the Great plateaus in any one year for the decade preceding 1870 was not less than 8,000,000, and even this vast number seems to be an underestimate. The great slaughter of the southern herd occurred from 1872 to 1874, and of the northern herd ten years later. In 1889, as stated by Hornaday, the bisons running at large in North America numbered but 635. In 1902 the number of bison in the United States was reported to be 800, the increase being due to protection extended to the herds, and perhaps also to a more accurate count. _Fur-Bearing Animals._--Of the animals of North America which are taken for their fur, the seal and sea-otter have already been referred to in connection with the brief review presented of the life of the continental shelf. As is well known, the colder regions of the earth are the ones which yield the most valuable furs, and in the fur trade of the world this continent, on account of its wide expansion at the north, has taken the leading place as a producer. In fact, the fur trade is a prominent feature in the history of America, and one whose followers experienced great vicissitudes and countless adventures.