LAWS OF TEMPERATURE CONTROL OF THE GEO- 
GRAPHIC DISTRIBUTION OF TERRESTRIAL 
ANIMALS AND PLANTS* 
ANNUAL ADDRESS BY VICE-PRESIDENT 
DR C. HART MERRIAM 
The tendency of animals and plants to multiply beyond the 
means of subsistence and to spread over all available areas is well 
understood. What naturalists wish to know is not how species 
are dispersed, but how they are checked in their efforts to over- 
run the earth. Geographic barriers are rare, except in the case 
of oceans, and since even these were formerly bridged at the north, 
another cause must be sought. This has been found in the group 
of phenomena commonly hidden under the word climate, and 
nearly a century ago it was shown by Humboldt that tempera- 
ture is the most important of these climatic factors. 
In the northern hemisphere animals and plants are distributed 
in circumpolar belts or zones, the boundaries of which follow lines 
of equal temperature rather than parallels of latitude. They 
conform in a general way, therefore, with the elevation of the 
land, sweeping northward over the lowlands and southward over 
the mountains. Between the pole and the equator there are three 
primary belts—Boreal, Austral and Tropical—each of which may 
be subdivided into minor belts and areas. In the United States 
the Boreal and Austral regions have each been split into three 
secondary transcontinental zones. The Boreal are known as the 
Arctic, Hudsonian and Cunadian ; the Austral as the Transition, 
Upper Austral and Lower Austral. The subordinate faunas and 
floras need not be here considered. 
* The present abstract of the principal results of an investigation carried 
on under the Department of Agriculture is here published by permission 
of the Honorable J. Sterling Morton, Secretary of Agriculture. The tem- 
perature data have been furnished by the United States Weather Bureau, 
a branch of the Department of Agriculture. A preliminary announce- 
ment of results was made by the author before the Philosophical Society 
of Washington May 26, 1894, 
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