SOURCES OF CHANGE 51 



new conditions as a species pushes out farther and 

 farther from its original center. 



Another source of environmental change whose 

 value must be extreme is gradual modification over 

 long periods of time as a result of geological 

 transformations. Elevation of a land mass may- 

 substitute the extreme diurnal fluctuations of high 

 altitude for the fairly even temperature of low 

 areas, or depression may result in the intolerable 

 climate of Death Valley. The thrusting up of a 

 new mountain range may bring a new abundance 

 of rainfall on one side and reduce the neighboring 

 lands on the other to aridity. The geological his- 

 tory of North America shows the past occurrence 

 of tropical and arctic conditions where it is now 

 temperate. Moreover these are not fleeting 

 changes. As construed by human concepts of time 

 they take place slowly and endure long. Within 

 human experience and no doubt for a vastly longer 

 time our continent has been temperate; the whole 

 world, in fact, has had a fairly constant distribution 

 of climatic conditions during recorded history. 



Fluctuations of the heritage are more elusive. 

 The heritage, in the first place, is a variable con- 

 cept. In the individual it is limited by the par- 

 ticular assortment of characters for which deter- 

 miners were received from the parents; in the 

 species it is the entire range of characters possible 

 to all of the individuals that may be produced. 



