482 THE EVOLUTION OF LIFE. 



siderablc, yet the land-gasteropods arc more closely allied to 

 them than to any other Mollusca. Similarly, the two highest 

 orders of crustaceans have their species which live occasion- 

 ally, 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 

 habitats expose them to frequent changes of media; how 

 some of the higher kinds so circumstanced, show a consider- 

 able adaptation to both media; and how these amphibious 

 kinds are allied to kinds that are mainly or wholly terres- 

 trial; we shall see that the migrations from one medium to 

 another, which evolution pre-supposes, are by no means im- 

 practicable. With such evidence before us, the assumption 

 that the distribution of the Vertebrata through media so dif- 

 ferent 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. The 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 few days 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 in- 

 habiting such waters -have, in various degrees, powers of 

 meeting these contingencies. The contained fish either bury 

 themselves in the mud when the dry season comes, or ramble 



