ANIMAL AND ITS ENVIRONMENT 465 



desert associations. They have, however, one characteristic in com- 

 mon — a great deficiency in available water. Plants must guard 

 against water loss and conserve water between the infrequent moist 

 periods. Some of the adaptations which meet such conditions are; 

 reduction of leaves (thorns), fleshy stems, thick cuticle, extensive 

 underground organs, etc. Animals conserve water by remaining 

 underground or in sheltered places during the hotter parts of the 

 day and coming out only at night or at dusk. Reptiles and birds 

 are able to reduce water loss by the absence of sweat glands and 

 by giving off their nitrogenous excretion in the form of uric acid 

 which does not need to be dissolved in water. 



Adaptation 



From what has been said above, it appears that each particular 

 set of climatic and other environmental conditions accommodates 

 a particular group of plants and animals. The activities and struc- 

 ture of these animals are such as enable them to survive best under 

 the conditions in which they are found. The barren ground caribou 

 is not fitted for life on the central grassland area nor is the bison 

 able to survive on the tundra. Each is said to be adapted to the 

 particular conditions under which it exists. Many so-called adap- 

 tations, however, seem to be worthless. One might think, for ex- 

 ample, that the shovellike brow tines of the caribou would be a 

 remarkable adaptation for shoveling snow from the vegetation on 

 which the animal must feed during the winter — ^but the antlers are 

 shed in the late autumn and do not grow out again until the fol- 

 lowing spring. One must be careful not to assign adaptive func- 

 tions to animal structures without careful consideration of the habits 

 and life histories of the animals concerned. 



Succession 



It must not be thought that, within the areas of the great climatic 

 formations, there is entire uniformity in the communities of plants 

 and animals. This is far from true. There exist minor differences 

 of climate, soil, and physiographic conditions and of biological his- 

 tory which result in differences in plant and animal population. 

 There are always areas which, for example, have been denuded by 

 physiographic or other processes. Fire, fiood, and human utilization 

 are among the more common influences which may cause partial or 



