Since Richardson’s time the mainland form of western Canada has been discussed by many zoologists and explorers. The most comprehensive account of its life history hitherto published is that by Seton (1929, +3+: 95-135),[1] whose personal experience was gained in the region of Artillery, Clinton-Colden, and Aylmer lakes. Dearth of adequate material (particularly from the type locality or adjacent areas) makes it all but impossible to determine whether there is any significant geographical variation between the herds of central Mackenzie and those of Keewatin. [Footnote 1: This statement, written long before the appearance of Banfield’s work of 1954, no longer applies.] The foremost objective of an expedition I made in 1947 to Nueltin Lake, in southwestern Keewatin, was a study of the Barren Ground Caribou. The expedition was supported by the Arctic Institute of North America, with funds supplied by the Office of Naval Research. My headquarters were at the little Windy River trading post, at the northwestern extremity of Nueltin Lake (map 1). There, for a period of six months, I enjoyed the fine hospitality of Charles Schweder and Fred Schweder, Jr. They had lived on intimate terms with the Caribou during most of their youthful lives, and they freely shared with me the knowledge they had thus gained concerning the ways of life of these wonderful creatures. They secured nearly all the specimens that went into my collection. The three other residents of the post also deserve my gratitude for their general helpfulness and friendliness; they were 10-year-old Mike Schweder (brother of Charles and Fred), 15-year-old Anoteelik (an Eskimo boy), and the latter’s sister, 5-year-old Rita. In a previous paper (1953) I have endeavored to express to various officials and friends my sincere appreciation of their courtesy and generosity in furthering the work of the expedition; and I can scarcely forbear to repeat here the names of at least a few of them: Dr. A. L. Washburn, at that time executive director of the Arctic Institute of North America; Mr. R. A. Gibson, deputy commissioner of the Administration of the Northwest Territories; and Mr. G. W. Malaher, director of the Game and Fisheries Branch, Manitoba. For the loan of a motion-picture camera, which secured for me some extremely gratifying scenes of the migrant hosts on the Barrens, I am greatly indebted to Mr. William C. Morrow. Dr. Ralph S. Palmer has kindly read, and made helpful comments upon, a preliminary draft of the present report. Through the courtesy of the American Museum of Natural History, the United States National Museum, and the United States Fish and Wildlife Service, I have been able to examine important comparative material in their collections. A grant from the National Science Foundation has enabled me to carry the investigation through to completion. MIGRATIONS