Causes Controlling Distribution. 45 



forms whose fossil remains have been found in many parts of 

 the United States have hot been able to return. The same is 

 true of plants, for the palms, tree-ferns, and numerous other 

 tropical types that formerly ranged over much of our country 

 are now either altogether extinct or exist only in the tropics. 



The llama and many plants now inhabiting the Andes may 

 be looked upon as representing a class of cases in which Boreal 

 forms were driven so far south that they actually reached the 

 great mountain system of South America and spread southward 

 over its elevated plateaus and declivities to the extreme end of 

 the continent in Patagonia and Terra del Fuego. This fact has 

 been long recognized by botanists. 



The paleontologic history of the earth shows that many groups 

 now unknown came into existence from preceding groups, 

 gradually attained a maximum development, and as gradually 

 passed away ; but there are few records of breaks in the geologic 

 series, or of disturbances of any kind from the earliest appear- 

 ance of life to the present time, that have resulted in the destruc- 

 tion of so many types as the cold of the Glacial epoch. 



CAUSES CONTROLLING DISTRIBUTION. 



It is now pretty generally conceded that temperature and 

 humidity are the chief factors governing the distribution of life, 

 and that temperature is more potent than humidity. Illustra- 

 tions of this law have been already given in contrasting the 

 humid and arid elements of the several zones with the zone 

 elements as limited by temperature, and it has been found in the 

 case of mammals and birds that the effects of temperature, esti- 

 mated numerically, are more than three times greater than the 

 effects of humidity upon genera, and many times greater upon 

 the higher groups. 



Authors differ as to the exact period during which tempera- 

 ture exerts the greatest influence, but there can be little doubt 

 that for both animals and plants it is the season of reproductive 

 activity, and hence varies inversely with latitude and altitude. In 

 high arctic latitudes this period is very brief, while in the humid 

 tropics it seems to extend over nearly if not quite the whole year.* 



Whether the temperature in question is the mean of a certain 



* This was pointed out by the author in North Am. Fauna No. 3, Sep- 

 tember, 1890, pp. 26-27. 



