SITUATION. 85 



breathe on land, but could not accomplish progressive mo- 

 tion. 



But even among the aquatic animals, which are less in- 

 fluenced in their distribution by temperature, and even 

 food, than those which reside on land, there are peculiar 

 arrangements connected with the reproductive system, 

 which exercise a powerful controul over them. With some 

 species, a constant residence in salt water is suitable to 

 every function ; but with others, it is indispensably requi- 

 site to have occasional access to fresh water. Thus the 

 salmon, smelt, and eel, which perform all the functions of 

 motion, perception, and digestion, in the sea, are impelled 

 to quit it during the season of generation, and repair to 

 fresh water rivers and lakes, where they deposit their 

 spawn, and where the fry sojourn for a short period. To 

 these aquatic animals both fresh and salt water are neces- 

 sary, so that their distribution is restricted to those regions 

 where both are accessible. The migrations which these 

 animals perform, appear to depend on the condition of the 

 reproductive system, influenced by the obstacles or facili- 

 ties for changing place, which the varying state of the fresh 

 waters present. 



Many quadrupeds and birds, which are able to execute 

 easily the conditions of progressive motion, and obtain an 

 abundant supply of food, in fresh or salt water, are com- 

 pelled to return to the land at regular intervals, for the 

 purposes of rest. This arrangement is exhibited in seals, 

 otters, and many birds, at all seasons, These animals, 

 consequently, cannot reside at great distances from land, 

 however suited the great ocean may be with regard to 

 temperature and food. In these animals the arrangements 

 of the reproductive system likewise compel them to fre- 

 quent the land during the breeding season. The term 

 Ampltibijus is usually applied to animals such as are now 



