No. 479] EXTINCTION OF MAMMALIA 



781 



Merriam obtained from extensive comparison of temperatures and 

 distribution 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 reproduction. Conversely animals and 

 plants (Upper Austral, Transition, and Boreal types coming from 

 the North) are restricted in southward distribution 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. 



(2) Ilumidity, observes Merriam, is a less potent factor than 

 temperature in limiting the distribution of the ^Mammalia of North 

 America.^ (a) Many genera adapted to certain restrictions of 

 temperature zones range east and west completely across the 

 American continent inhabiting alike the humid and arid sub- 

 divisions but no genus adapted to certain restrictions of humidity 

 ranges north and south across the temperature zones, (h) Thus 

 humidity governs the details of distribution of a few species of 

 mammals within the temperature zones. 



Lowering of Temperature and Diminished FeriUHij as a Cause of 

 Extinction. — Since the favorable inlhience of higli mean tem- 

 perature on fertility and reproduction is well illustrated in the 

 reproductive organs of birds and in the early age of reproduction 

 and increased fertility of the human species toward the equator, 

 and Muce there exist th(\se K)w- temperature barriers to reproduc- 

 tion, it is liighly probable that a secular lowering of temperature 

 may have repeatedly been a cause of extinction in the earth's 

 historv^; that certain niannnals may have resisted exposure to 

 cold or discovered new forms of food and yet sutl'ered extinction 

 through the subtle iidiihition of fertihty and reproduction. 



