Oftentimes environment modifies the physique of a people indirectly by imposing upon them certain predominant activities, which may develop one part of the body almost to the point of deformity. This is the effect of increased use or disuse which Darwin discusses. He attributes the thin legs and thick arms of the Payaguas Indians living along the Paraguay River to generations of lives spent in canoes, with the lower extremities motionless and the arm and chest muscles in constant exercise.[43] Livingstone found these same characteristics of broad chests and shoulders with ill-developed legs among the Barotse of the upper Zambesi;[44] and they have been observed in pronounced form, coupled with distinctly impaired powers of locomotion, among the Tlingit, Tsimshean, and Haida Indians of the southern Alaskan and British Columbia coast, where the geographic conditions of a mountainous and almost strandless shore interdicted agriculture and necessitated sea-faring activities.[45] An identical environment has produced a like physical effect upon the canoemen of Tierra del Fuego[46] and the Aleutian Islanders, who often sit in their boats twenty hours at a time.[47] These special adaptations are temporary in their nature and tend to disappear with change of occupation, as, for instance, among the Tlingit Indians, who develop improved leg muscles when employed as laborers in the salmon canneries of British Columbia. [Sidenote: Effects of climate.] Both the direct and indirect physical effects of environment thus far instanced are obvious in themselves and easily explained. Far different is it with the majority of physical effects, especially those of climate, whose mode of operation is much more obscure than was once supposed. The modern geographer does not indulge in the naive hypothesis of the last century, which assumed a prompt and direct effect of environment upon the form and features of man. Carl Ritter regarded the small, slit eyes and swollen lids of the Turkoman as "an obvious effect of the desert upon the organism." Stanhope Smith ascribed the high shoulders and short neck of the Tartars of Mongolia to their habit of raising their shoulders to protect the neck against the cold; their small, squinting eyes, overhanging brows, broad faces and high cheek bones, to the effect of the bitter, driving winds and the glare of the snow, till, he says, "every feature by the action of the cold is harsh and distorted."[48] These profound influences of a severe climate upon physiognomy he finds also among the Lapps, northern Mongolians, Samoyedes and Eskimo. [Sidenote: Acclimatization]