160 PROBLEMS OF BIRD LIFE. 



zones or belts around it all the way up to the top, each 

 inhabited by slightly different trees, insects, birds, and mam- 

 mals, until we reached the summit of perpetual ice, barren 

 of life and vegetation. 



We may regard the earth itself as such a mountain, its 

 ice-capped pole the summit, surrounded by zone below zone 

 of vegetation each more luxuriant than the one above it 

 until we reach the equator, or base of our world mountain. 

 There is a curious similarity between the belts on a moun- 

 tain and the zones of the earth. Very often on the hills 

 and mountains of New England I have picked the spicy 

 mountain cranberry, the goosefoot potentilla with its starry 

 flowers, or the scrubby little Corema, and have recognized the 

 land I was on as like the coast of Labrador; it was as if 

 I had travelled north five hundred miles instead of climbing 

 up half a mile. Still more remarkable is the case of higher 

 mountains farther to the south. On San Francisco Mountain 

 in Arizona, which rises 12,800 feet above the sea, have been 

 found Arctic plants identical with those of the extreme North. 1 

 Nine species found on this mountain have proved to be pre- 

 cisely similar to those brought by General Greely from Lady 

 Franklin Bay, latitude eighty-two degrees north. By climb- 

 ing a mountain two miles and a half in height, we would be 

 able to see plants growing for which we must go more than 

 three thousand miles if we journeyed due north. Thus the 

 tops of high mountains give us an arctic climate. 



Now we see why it is that northern birds are sometimes 

 found- far to the south. They follow down the mountain 

 peaks and find at different elevations the zone which gives 

 them the climate they would naturally seek in the north. 

 Or if they have wintered south and would return in summer 

 1 Davis, Elementary Meteorology, p. 343. 



