18 NORTH AMERICAN FAUNA 61, FISH AND WILDLIFE SERVICE 



brate fauna. An exceptionally nutritious marine food source 

 should influence the vigor and size of the terrestrial vertebrates 

 of that region. 



The song sparrow's habitat in the Aleutians is the beach, and 

 it is probable that its food is largely of marine origin — the small 

 beach crustaceans, for example. Some other land birds, such as 

 winter wrens and pipits, feed to some extent on the beach. The 

 blue foxes feed chiefly on marine life. 



The case of the Alaska brown bears is not so clear, though for 

 a part of the summer they comb the beaches and live extensively 

 on salmon, which are nourished in the sea. One wonders, also, 

 if a certain type of food may, with other factors, help to encourage 

 melanism (as in the jaeger), or darker shades of color, as in some 

 of the other birds. It is generally accepted that a humid habitat 

 produces dark coloration. It is not certain that this tendency, 

 as well as melanism, is encouraged by rich food. 



This is, of course, pure speculation, yet the significance of a 

 food chain from the sea to the higher vertebrates on the adjacent 

 land may be worthy of earnest study. There are many birds that 

 have not responded to environmental influence. The Aleutian song 

 sparrow has not developed dark pigmentation to an unusual de- 

 gree. The northern form of the fork-tailed petrel, though averag- 

 ing larger in the Aleutians, apparently is paler than those in 

 southeastern Alaska. Also, it must be considered that the in- 

 terior Alaska and Yukon caribou, as well as the Alaskan moose, 

 which have no direct connection with the sea, are the largest on 

 this continent. But these examples suggest that there is some- 

 thing in the environment — favorable food, humidity, or other 

 stimuli — that tends to produce dark pigmentation and large size. 

 This is an important challenge to future investigation and 

 understanding. 



ECOLOGICAL CLASSIFICATION 



By the usual standards of life-zone allocation, the Alaska Pen- 

 insula and Aleutian Islands would fall chiefly in the Arctic Zone. 

 A part of the Kodiak-Afognak Island group supports tree growth, 

 and forests encroach on the base of Alaska Peninsula to the 

 vicinity of Mount Katmai. Therefore, these locations would mark 

 the limit of the Hudsonian Zone. However, we find the life-zone 

 classification here to be far from simple. There are probably a 

 number of physiographic and oceanic reasons for this situation. 



There are serious difficulties in the interpretation of life zones 

 in the Aleutians that should be considered. The lack of trees 



