NATllRAr. HISTORY 



Typical xerophytic plants of the desert areas. 



Hau'oTtIt 



water supply, mesophytes ; and those which associate in desert condi- 

 tions, xewphytes. Animals which live in the water are said to be aquatic, 

 those on land terrestrial, while those that live both on land and in 

 water are called amphibious. Animals and plants associated in still 

 water are quite different from those in running water, while different 

 types of plants and animals are found close to shore, in deep water, in 

 rapid water, on rocky shores or on sandy shores, in salt or in fresh 

 water, and in tidal pools or on the sand. Everywhere we find dif- 

 ferent associations of plants and animals. Many explanations are 

 given, but no one explains everything. One investigator, Merriman, 

 emphasizes temperature as an all-important factor ; Walker gives 

 atmospheric pressure ; Heilprin, food ; and Shelford, in recent experi- 

 ments, indicates that the conditions under which an animal breeds 

 may greatly influence its distribution. He experimented with tiger 

 beetles, using different soils such as clay, clay and humus, humus, 

 humus and sand, and pure sand. The beetles lay their eggs only in 

 moist soil, therefore this factor was constant with all the soils. In 

 this experiment the soils were also placed at a level and on slants. 



