50 THE PROBLEMS OF EVOLUTION 



from sage brush only a foot or two above the 

 ground and seen them die when dropped onto a 

 surface nearby which was without protection from 

 the rays of the sun. At night the same hmitation 

 of the air permits rapid dissipation of heat, both 

 from the earth and from the body, and contrast- 

 ingly low temperatures result. Water supply fluc- 

 tuates with rainfall, which is conditioned by many 

 factors. In central California and similar regions 

 the summer months find practically desert condi- 

 tions while winter brings abundant rainfall. The 

 organic environment is no less variable. The life 

 cycles of plants and the migrations of animals 

 bring about changes from season to season in food 

 supply and other less direct contacts. And the 

 internal environment varies constantly, but in a 

 degree which is in inverse ratio to the degree of 

 independence from the external. 



The locomotion of animals and gradual dis- 

 persal of plants may also bring them into contact 

 with different conditions of environment as they 

 invade new regions. Other things being normal, 

 animals probably seek congenial surroundings and 

 plants develop better under favorable conditions, 

 hence their contacts are not likely to differ from 

 those established under fluctuating conditions in 

 one environment while such conditions are avail- 

 able. It is more than likely, however, that dis- 

 persal is accompanied by gradual adjustment to 



