INTRODUCTION 19 



energy of sunlight they incorporate these com- 

 pounds into the food supply of most of the organic 

 world and any factors tending to modify the avail- 

 able supply of these several materials inevitably 

 bring about adjustments of the flora. Trees do not 

 grow on prairie areas, in some places because the 

 prevailing winds are too dry to permit their de- 

 velopment, favoring only the growth of low xero- 

 phytic vegetation. Elsewhere great altitude brings 

 about rapid fluctuations of temperature and a 

 short growing season; here again a characteristic 

 flora prevails. 



Animals respond to the physical environment no 

 less certainly than plants, although their response 

 involves less fundamental needs. A conspicuous 

 evidence of animal response is the adjustment of 

 the circulation to low pressure at high altitude and 

 the peculiar effect of the sudden release of the 

 human body from high pressure. Anyone who has 

 attempted normal activity soon after moving to 

 an altitude of ten thousand feet above sea level 

 will realize that he is not adjusted to the prevail- 

 ing conditions, but after a few weeks the increase 

 of red corpuscles compensates for the low concen- 

 tration of oxygen in the air. And after the appear- 

 ance of a popular account of the last submarine 

 disaster few people can be unaware of the neces- 

 sity for bringing divers gradually back to surface 

 pressure after they have worked at a considerable 



