CHAPTER FIFTY-FIVE 



The life 

 zones of 

 North 

 America 



Isotherms 



LIFE ZONES 



1. EVERY one knows that there are in North America 

 very different regions, producing special kinds of plants 

 and animals. Not only do the native or wild products 

 of these regions differ, but of course also the crops and 

 conditions affecting human life. Thus we have the 

 corn belt and the cotton belt, the wheat country and 

 the grazing country, etc. Many years ago Dr. C. 

 H. Merriam, then of the United States Department of 

 Agriculture, undertook to make a careful study of this 

 subject, in order to determine general principles or 

 laws which might be of scientific and practical value. 

 He found out that, broadly speaking, the whole country 

 could be divided into a number of belts or zones, which 

 he called life zones, each distinguished by its fauna and 

 flora. This was not in itself a new idea, but it had 

 never before been worked out in detail, with such a 

 mass of data. The several zones did not, of course, 

 differ entirely in their products ; but when the observer 

 took note of a number of different plants or animals 

 in any locality, he usually had little difficulty in de- 

 termining its zonal position. Sometimes the transi- 

 tion was quite abrupt, as for instance at the limit of 

 trees in the north or on mountains, and less con- 

 spicuously in the limitation northward of tropical 

 plants which cannot endure frost. 



2. These zones owe their biological differences almost 

 entirely to climate. From south to north, and from 

 lower to higher altitudes, the climate becomes colder. 

 The isotherms are lines running across the country, 

 marking the same temperature for the year or any 

 particular part of it. It seems at first very simple 



454 



