394 THE EVOLUTION OF LIFE. 



crustaceans have their species which live occasionally, or 

 almost entirely, out of the water : there is a kind of lobster 

 in the Mauritius which climbs trees ; and there is the land- 

 crab of the West Indies, which deserts the sea when it reaches 

 maturity, and re- visits it only to spawn. Seeing, thus, how 

 there are many kinds of marine creatures whose habitat 

 habitually exposes them to changes of media ; how some of 

 the higher kinds so circumstanced, show a considerable adapt- 

 ation to both media ; and how these amphibious kinds are 

 allied to kinds that are mainly or wholly terrestrial ; we 

 shall see that the migrations from one medium to another, 

 which evolution pre-supposes, are by no means impracticable. 

 With such evidence before us, the assumption that the dis- 

 tribution of the Vertebrata through media so different as air 

 and water, may have been gradually effected in some analogous 

 manner, would not be altogether unwarranted, even had we 

 no clue to the process. We shall find, however, a tolerably 

 distinct clue. Though rivers, and lakes, and pools, 



have no sensible tidal variations, they have their rises and 

 falls, regular and irregular, moderate and extreme. Especially 

 in tropical climates, we see them annually full for a certain 

 number of months, and then dwindling away and drying up. 

 This drying up may reach various degrees, and last for various 

 periods : it may go to the extent only of producing a liquid 

 mud, or it may reduce the mud to a hardened, fissured solid ; 

 it may last for a day or two or for months. That is to say, 

 aquatic forms which are in one place annually subject to a 

 slight want of water for a short time, are elsewhere subject 

 to greater wants for longer times : we have gradations of 

 transition, analogous to those which the tides furnish. Now 

 it is well known that creatures inhabiting such waters, have, 

 in various degrees, powers of meeting these contingences. 

 The contained fish either bury themselves in the mud 

 when the dry season comes, or ramble in search of other 

 waters. This is proved by evidence from India, Guiana, Siam, 

 Ceylon ; and some of these fish, as the Anabas scandens, are 



