of the Snake Indians inhabiting the Rocky Mountain deserts distinguished
these clans from the tall buffalo-hunting tribes of the plains.[42] Any
feature of geographic environment tending to affect directly the
physical vigor and strength of a people cannot fail to prove a potent
factor in their history.

[Sidenote: Physical effects of dominant activities.]

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.