152 University of California Publications in Zoology, ["^ol. 7 



to explain satisfactorily, though in some instances the causes 

 of the present conditions are fairly obvious. 



The relationships of the vertebrate fauna of the Sitkan dis- 

 trict are for the most part distinctly with the mainland region 

 to the southward, the temperate climate of the coast having 

 permitted this northern extension of the ranges of many species of 

 birds and mammals. The greater part of the area included is 

 composed of islands, the strip along the mainland between the 

 mountains and the sea being narrow and restricted. Though the 

 fauna of the various islands has, as a rule, apparently been 

 derived from the adjacent mainland, there are several exceptions, 

 as noted beyond, some of them difficult to understand. 



The northern islands of the group are separated from the more 

 southern ones by broad, deep channels, and many of the mammals 

 of these islands are more closely related to more northern main- 

 land forms. Of the southern islands those lying closest to 

 the mainland have the greatest number of mainland species. 



Island forms of several species are somewhat smaller than 

 the mainland races, but it is remarkable that on the westernmost 

 islands, those farthest from the mainland, these same species 

 should be represented by exceedingly large races. 



Climatic and physiographic conditions are quite uniform 

 throughout the whole of the area under consideration, which is 

 a region of rough, precipitous mountains and impenetrable 

 forests, and is likewise noted for its copious rainfall and com- 

 paratively mild climate. The most noticeable local climatic 

 variation is between the mainland and the more western islands. 

 The winter climate of the mainland coast, though much milder 

 than that of the region east of the mountains at the same latitude, 

 is more rigorous than that of the islands. The islands nearest the 

 coast, as Revillagigedo, are on their east sides also subject to 

 severe winter weather. One of the results is that on the mainland 

 the heavy precipitation of the region takes the shape of snow 

 which covers the ground to such a depth as to render impossible 

 the presence of animals like the deer. Despite the general 

 uniformity of conditions it is interesting to note that with one 

 or two possible exceptions there is no species of mammal that 

 ranges unchanged throughout the whole of the region. With 



