i THE WEALTH OF LIFE 7 



carry small animals and the larvae of larger animals on 

 their muddy feet from one basin to another. Others are 

 borne by the wind, and changes of land-level may bring 

 different river-beds into communication. In a true lake, 

 as in the sea, it is necessary to distinguish three faunas 

 -of the shore, of the surface, and of the deep water. 



As we review the series of animals from the simplest 

 upwards, we find a gradual increase in the number of 

 those which live on land. The lowest animals are mostly 

 aquatic the sponges and stinging-animals wholly so ; 

 worm-like forms which are truly terrestrial are few com- 

 pared with those in water ; the members of the starfish 

 group are wholly marine ; among crustaceans, the wood- 

 lice, the land-crabs, and a few other dwellers on the 

 land, are in a small minority ; there are many insects 

 which spend their larval life or even their whole life 

 in the water, but their numbers are small compared with 

 the terrestrial and aerial hosts ; among spiders aquatic 

 forms are quite exceptional ; and while the great majority 

 of molluscs live in water, the terrestrial snails and slugs 

 are legion. In the series of backboned animals, again, 

 the lowest forms are wholly aquatic ; an occasional fish 

 like the climbing-perch is able to live for a time ashore ; 

 the mud-fish, which can survive being brought from 

 Africa to Europe within its dry " nest : of mud, has 

 learned to breathe in air as well as in water ; the am- 

 phibians really mark the transition from water to dry 

 land, and usually rehearse the story in each individual 

 life as they grow from fish-like tadpoles into frog- or 

 newt-like adults. Among reptiles, however, begins that 

 possession of the earth, which in mammals is established 

 and secure. As insects among the backboneless, so 

 birds among the backboned possess the air, achieving in 

 perfection what flying fish, swooping tree-frogs and 

 lizards, and above all the ancient and extinct Pterodac- 

 tyls, have reached towards. Interesting, too, are the 

 exceptions non-flying terrestrial birds like ostriches, 

 non-flying aquatic penguins, aquatic mammals like the 

 whales, aerial mammals like the bats, and so on. 



But it is enough to emphasise the fact of a general 



