9 
Rocky Mountains back-bone south from northern British Columbia to 
across the boundary line. 
Ocean currents, also, have an important effect on the climatic conditions 
of the shores adjoining them. The fact that Land’s End in England, 
washed by the last energies of the warm Gulf stream, is in the same latitude 
as bleak Labrador, bathed by a cold Arctic current, well illustrates this. 
On the Pacific, the great Japan current sweeps against our coast and pro- 
longs Canadian zone conditions as far north as the latitude of southern 
Greenland, and moderates the climate of southern Alaska and the far- 
flung Aleutian islands to a surprising mildness. 
Besides these purely thermal controls of climate, there are other influ- 
ences. Important amongst these is rainfall. The west coast, bathed in 
winds moisture-laden from the warm sea, receives a copious rainfall and 
the vegetation is luxuriant and almost tropical in its profusion. These 
rain-laden winds, however, are condensed by the cold peaks of the coastal 
range and pass on over to the interior robbed of most of their moisture. 
Hence the valleys of British Columbia and the prairie regions beyond are 
in some cases almost arid in character. 
Other conditions are influential in affecting the distribution of bird 
life. The treeless prairies, with their dry uplands interspersed with myriads 
of shallow lakes and sloughs fed largely by the melting of the winter’s snow 
and in many cases strongly alkaline, attract an entirely different class of 
birds than do the heavily forested woodland to the north or east, or the 
mountainous country farther west. Another source of differentiation in 
bird life has been the north and south trend of the great western mountain 
ranges, forming routes for free extent of range up and down the continent, 
but limiting it east and west. Many species cross these mountains without 
difficulty, but others find them an almost complete barrier to free dispersal. 
Their effect has been to populate the regions west of the Rocky Mountain 
range with many species that have never found their way east. These 
mountains have also had the effect of breaking up the country into num- 
erous more or less isolated communities each with its own peculiar physical 
characteristics that have developed a large number of geographical or 
subspecific races. In consequence many species that show homogeneous 
characters from the Atlantic to the mountains break up into a number of 
special forms from the mountains westward. 
Taking the eastern forms as typical in the ordinary acceptance of the 
word, comparable birds of the prairie are slightly smaller and considerably 
paler in coloration, whereas on the humid Pacific coast they are larger and 
much darker in colour. Through these influences, therefore, we find in the 
west many subspecies of eastern forms. Comparatively few species range 
unmodified across the continent, many are represented east and west by 
two or more subspecies showing greater or less differentiation, and in 
other cases they are replaced by closely allied species or are absent alto- 
gether. 
In noting these faunal divisions, however, it must be remembered that 
as far as birds are concerned these associations have to be based entirely 
upon breeding individuals. Birds travel so widely and along so many 
devious routes in their migration, that they may pass through several 
faunal areas in spring and autumn though breeding in only one. There- 
fore, in determining the faunal zone to which any given area should be 
referred, such transients must be disregarded. 
