A BIRD’S GEOGRAPHY AND ARITHMETIC. 187 
Geologists hint that in the long ago the Gulf waters 
extended north till they met those of Hudson Bay, 
and that our plains were once the bottom of a shallow 
ocean whose beaches were the highlands of the Appa- 
lachian and Rocky Mountain regions respectively. 
There is much in the distribution and migration 
of our birds to confirm this. At any rate, there are 
on our continent two great divisions of migrant birds, 
those from the east going south, usually on the east 
side of the Gulf, and passing on mostly by the West 
Indies, and those from the west going southwest of 
the Gulf and passing on by the Isthmus. 
Except in a few instances, which, like those of the 
bobolink, are comparatively recent, our great plains 
west of Missouri have been almost as complete a bar- 
rier to the mixture of the birds of the two regions as 
the original waste of waters was. 
Now these two masses of bird life show consider- 
able resemblance to each other, but frequently differ 
in genera and species. Often, however, only the 
slightest variations in coloration are evident between 
Eastern and Western species. In some instances (as 
the flickers, meadow larks, ete.) it is evident that they 
have intergraded across the plains, but in others the 
line of kinship more likely runs around by South 
America or by the arctic landed regions, where the 
intergradations were made. In a few cases, some 
Western birds show their kinship from Asia, by the. 
way of the Aleutian Isles say, and some Eastern birds 
have a cousinly line running across to Europe via 
Iceland. 
