2 TERRESTRIAL AND AQUATIC ANIMALS. [CH. I 



fresh waters themselves gradually pass into salt at the 

 mouths of rivers ; while a marsh, particularly one that is 

 subject to periodical drying up, is to some extent inter- 

 mediate between the land and the water. 



Corresponding to this absence of hard and fast lines in 

 inorganic nature we find a similar absence of definiteness 

 in the animals which frequent the waters, the dry land, 

 and the transitional areas. The duck tribe are equally at 

 home when swimming in a lake and when flying from one 

 pool to another. There are even ducks which perch on 

 trees. Closely allied Crustacea may be marine, estuarine, 

 fresh water or terrestrial in habit. Certain sharks, nor- 

 mally marine, ascend rivers for some distance, and the 

 Manatee of the west coast of Africa and the east coast of 

 America is quite as much at home when browsing upon 

 the marine algae of the coasts of those continents as when 

 living in their rivers. Though we may broadly separate 

 animals into terrestrial and aquatic, there is no large 

 group of animals which is exclusively terrestrial. Even 

 insects which approach nearest to this condition have 

 plenty of aquatic representatives ; there is even a peculiar 

 genus of bugs, Halobates, which inhabits the open sea far 

 from land. 



On the other hand, there are groups which are purely 

 aquatic, and even some which are only marine, being never 

 found in fresh water ; but these are few. Among Verte- 

 brates fishes are the only group which can be said to be 

 almost absolutely aquatic ; and here too there are some 

 slight exceptions. The well-known Climbing Perch, Anabas 

 scandens, can with impunity leave the streams which it 



