504 THE AGE OF MAMMALS 



Temperature control of fertility and reproduction. — Merriam ^ has directed 

 attention to one of the physiological effects of a lowering of temperature, 

 namely, its influence upon diminished or increased fertility and the rate of 

 reproduction in what he has called the 'law of temperature control.' This 

 he has stated as follows: temperature by controlling reproduction prede- 

 termines the possibilities of distribution; it fixes the limits beyond which 

 species cannot pass ; it defines broad transcontinental barriers within which 

 certain forms may thrive if other conditions permit, but outside of 

 which they cannot exist, be the other conditions never so favorable (because 

 the sexes are not fertile). 



Temperature and geographic range. — In discussing how species are 

 checked in their efforts to overrun the earth Merriam points out that more 

 important than geographic barriers are the climatic barriers (as observed 

 by Humboldt), and of these that temperature is more important than 

 humidity. First, in 1892, this author attempted to show ^ that the distribu- 

 tion of terrestrial animals is governed less by the yearly isotherm or mean 

 annual temperature than by the total rather than the mean temperature 

 during the period of reproductive activity and of growth (adolescence). 

 This reproductive period in the tropics extends over many months or nearly 

 the whole year, and within the Arctic Circle and summits of high moun- 

 tains is of two months or less duration. Later, in 1894, results which Mer- 

 riam * obtained from extensive comparison of temperatures and distribu- 

 tion justified the belief that animals and plants (Lower Austral and tropical 

 types coming from the South) are restricted in northward distribution by 

 the total quantity of heat during the season of development and reproduc- 

 tion. Conversely animals and plants (Upper Austral, Transition, and 

 Boreal types coming from the North) are restricted in southward distribu- 

 tion by the mean temperature of a brief period covering the hottest part 

 of the year. Thus in the Transition Zone, Boreal and Austral types mingle 

 in the equable climate of the Pacific coast of California, while they are 

 sharply separated by the inequable extremes of cold and heat of the interior 

 continental plateau. 



It follows from these observations that animals forced out of their 

 natural habitat may become extinct through infertility. 



Influence of cold and snow on food supply and choice of food. — It is prob- 

 able that during the Glacial Period the great winter snow blanket cover- 

 ing the natural food supply, rather than the direct influence of the cold 

 itself, was one of the chief causes of extinction. The death of great numbers 

 of animals from hunger or starvation through the covering of food during 

 the winter season under heavy layers of snow is commonly observed among 



1 Merriam, C. H., Laws of Temperature Control of the Geographic Distribution of Terres- 

 trial Animals and Plants. Nat. Geogr. Mag., Vol. VI, Dec. 29, 1894. 



2 Merriam, C. H., The Geographic Distribution of Life in North America with Special 

 Reference to the Mammalia. Proc. Biol. Soc. Washington, Vol. VII, Apr. 13, 1892, pp. 45-46. 



