472 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



those of Arctic regions or deserts, would afford the strongest chance 

 of discovering the fundamental laws governing the interrelations of 

 animals and therefore the regulation of their nmnbers" (ibid., p. 

 921). Huxley (Elton, 1939, Introduction) says that Elton has been 

 fortunate in having field experience in the Arctic, where the ecological 

 web of life is reduced to its simplest and where complexity of detail 

 does not hide the broad outlines. It seems valid to assume that con- 

 ditions affecting animal life in simpler communities would also in- 

 fluence the animal's host, man. 



The first of the environmental factors controlling the occurrence 

 and numbers of animals in the Arctic to be considered are the climatic 

 conditions. These, of course, do directly act upon animals, but a 

 great deal of this influence is felt indirectly through plants leading 

 back to the ultimate source of energy, the sun. As far as the animals 

 are concerned, however, the whole character of the climate is deter- 

 mined by the plants. Difl'erent areas of plant communities may set 

 apart animal zones or "life zones," such as the habitat of the Barren 

 Ground caribou on the north slope of Alaska, which is set apart from 

 the moose zone farther to the south and closer to the timber line. 

 This distinction of one life zone as compared to another may be 

 the difference of one kind of human ecology from another. This 

 should be qualified somewhat. Humans are but indirectly tied to 

 the plant life of their habitats. Therefore, they may change habitat 

 at will. The latter course rests upon the proposition that other means 

 of supplanting previous economy are to be found and that no hostile 

 peoples or geograpliical barriers exist to thwart migration. 



We are quite certain that man began to enter the New World some 

 time or times in the latter part of the Pleistocene epoch and the begin- 

 ning of the Recent epoch in geologic time. Several successive glacia- 

 tions — at least four — alternating with thaws, capped the northern end 

 of the earth principally in the more elevated regions. In North 

 America, man's entry was subsequent to the last glaciation, which oc- 

 curred about 25,000 or more years ago. It is presumed that there 

 remained a great deal of ice on portions of the earth's surface when 

 Early Man came to America. This ice undoubtedly had a bearing on 

 the routes used by the migrating incomers (Roberts, 1940, p. 102). 

 The fossil records show clearly that there had been an interchange 

 of fauna between Asia and America. Man evidently journeyed in 

 one direction only, eastward to this hemisphere, and probably over 

 a land bridge during the earlier stages. 



As Ivar Skarland (n. d., p. 126) succinctly pointed out, a land bridge 

 was present only during glacial stages. This land bridge was more 

 literally than figuratively a truth, to judge by the strength of the argu- 

 ments in its favor. The now-familiar hypothesis is that the Ice Age, 

 in locking the water in glaciers, lowered the sea level enough to permit 



