This fundamental difference in construction must be recognized in comparisons of Eskimo and North American Indian watercraft. Here, too, it might be observed that one should view with skepticism any claim that widespread similarity of certain structural practices is evidence of some ancient connection between types of canoes. In most cases these similarities were imposed by the working characteristics of the materials employed. Similarly, limitations in materials available for construction have their effect upon building techniques. The practice of employing pressure members in bark-canoe construction, particularly where birch bark was employed, was the result of the need to stretch this material by gentle and widespread pressure, whereas the skin cover could be stretched by the concentrated pull of stitching alone, or by force applied in a small area. Bark canoes built in areas where skin-kayak construction is carried on nearby show a greater rigidity of structure. Thus, in the lower Yukon Valley in Alaska the bottom frame of the canoes built there was a rigidly constructed unit, even though the side longitudinals were held in place by rib pressure alone. And it is reasonable to theorize that the Malecite, who through habit still employed bark-canoe construction practices in building their skin craft, would have eventually come to the Eskimo method of construction had conditions required them to use skins exclusively. [Illustration: Figure 213 LARGE MOOSE-HIDE CANOE of upper Gravel River, Mackenzie valley. (_Photo, George M. Douglas._)] _Appendix_ The Kayak Roll _John D. Heath_ The most extraordinary feat of kayak handling is the ability to right the craft after a capsize. This maneuver, called "rolling," is usually practiced by capsizing on one side and recovering on the other. Under emergency conditions, a kayaker will recover on whichever side is more convenient. When rolling, a kayaker wears a waterproof jacket having long sleeves and a hood. The waist, face, and wrist openings are fitted with drawstrings, so that when the waist opening is fitted over the cockpit rim, the kayak and kayaker become a waterproof unit. Thus equipped, the kayak is the most seaworthy craft of its size, this quality being limited only by the skill and stamina of the kayaker. The art of kayak rolling was highly developed in Alaska and Greenland. Eskimos in both of these regions depended upon seal hunting by kayak as a major part of their economy, hence the ability to roll was an important means of survival. Very little detailed information exists regarding Alaskan kayakers, but the Greenlanders have been the object of intensive study by ethnographers and explorers. The earliest detailed record of rolling was that of David Crantz, a European missionary, who in 1767 enumerated ten methods of rolling in his _History of Greenland_.[7] His description follows. [7] See bibliography.