68 



ECOLOGY 



Part II 



I Fig. 5.1. Abundance. Gannets nesting on ledges of Bonaventure Island, off the 

 coast of the Gaspe Peninsula, on rocks as high as a 20-story building. A gannet is 

 about the size of a duck. (Courtesy, Allan D. Cruickshank, from National Audu- 

 bon Society.) 



make classification possible. Animals may have two or four or more legs; 

 insects have six; spiders have eight; there are a hundred or more in millipedes. 

 The bones of the arms and legs of a man are arranged like the comparable 

 bones in the legs of a horse (Fig. 9.13), but in other ways the legs are differ- 

 ent. Such structures are correlated with the history of their environment, 

 human arms and legs with ancestors that climbed trees and the horse's legs 

 with ancestors that ranged the swamps and the plains. Various noses are 

 adapted to various functions in addition to smell; an elephant can give itself 

 a shower bath with its nose (Fig. 5.3). 



Sizes of Animals and the Environment. Animals of a given species vary 

 relatively little in size. Size, proportions, and structure of the body and en- 

 vironment are mutually related. Water lifts and supports weight as air does 

 not; boats can anchor and float but airplanes cannot poise in the air without 

 special devices. Many animals can swim, but few can fly. Aquatic animals are 

 often larger than their terrestrial near-relatives and literally lean on the water 



