276 PRINCIPLES OF ANIMAL BIOLOGY 



Not all animals in the same environment are adapted in the same way to 

 the conditions of that environment. Thus the manatee, whales and 

 porpoises, all of them aquatic mammals, do not have the fins of fishes, 

 and they breathe by means of lungs; and many terrestrial animals do not 

 have legs. The fact remains, however, that animals living in different 

 conditions are often structurally different, and these differences in struc- 

 ture may fit them for life in their respective environments. This does not 

 mean that all differences in structure are adaptive, for such universal 

 adaptation has not been established; but it is reasonably clear that animals 

 are often structurally adapted to particular conditions, and are prob- 

 ably never found in conditions to which their structural characters are 

 seriously ill-adapted. As more detailed examples of structural adap- 

 tations may be mentioned the suctorial disks on the toes of the tree- 

 inhabiting frogs, the webbed or lobate feet of ducks and other aquatic 

 birds, the tuberculate teeth of browsing animals like the elk, and the 

 teeth with fiat grinding surfaces in grazing animals like the bison. 



Animals may also be apparently adapted to particular environments 

 by their coloration. Whatever the explanation may be, many forms in a 

 particular environment have a coloration which tends to conceal them by 

 making them look like something in the environment. However, the 

 dependence of forms so colored upon their resemblance to specific things 

 in the environment has not been shown, and the resemblance may be 

 much less adaptive than commonly supposed. 



Physiological Adaptation. — The general physiological dependence of 

 animals upon the physical conditions which surround them has also long 

 been recognized, but the extent of this dependence and the difference 

 in the effect of the same conditions upon different forms is only now 

 coming to be appreciated. It is generally known for example that most 

 of the marine animals cannot live in fresh water, nor freshwater species 

 in salt water; that many northern animals are adversely affected by the 

 higher temperatures of the tropics, while many tropical animals cannot be 

 acclimated to temperatures which prevail in northern latitudes; that 

 species which thrive in regions of moist conditions many times cannot 

 live in desert areas; and so on. It is not, however, so generally known 

 that the dependence upon certain conditions is so close as to cause 

 differences in the local occurrence of a form in a very limited region 

 and in the purely local distribution of nearly related species. The data 

 show that the physiology of the animal is affected by many conditions of 

 the environment, and that variations in any of the conditions may be 

 measured from an optimum (most favorable condition) which may be 

 quite different for different species. Thus water containing 6 cc. of oxygen 

 per liter is said to be suitable for the brook trout while the Mackinaw 

 trout requires less and is found in water with but 1 cc. of oxygen per liter. 

 In one family of freshwater fishes the acidity of the water preferred 



