CHAP. I The Wealth of Life 7 



been that which led to the establishment of the greater part 

 of our freshwater fauna. Professor Sollas has shown with 

 much conclusiveness that the conversion of comparatively 

 shallow continental seas into freshwater lakes has taken 

 place on a large scale several times in the history of the 

 earth. This has been in all likelihood accompanied by the 

 transformation of marine into freshwater species. It is 

 thus, we believe, that our lakes and rivers were first peopled. 

 Many freshwater forms differ from their marine relatives in 

 having suppressed the obviously hazardous free-swimming 

 juvenile stages, in bearing young which are sedentary or in 

 some way saved from being washed away by river currents. 

 Minute and lowly, but marvellously entrancing, are 

 numerous Rotifers, of which we know much through the 

 labours of Hudson and Gosse. These minute forms are 

 among the most abundant tenants of fresh water, and their 

 eggs are carried from one watershed to another on the 

 wings of the wind and on the feet of birds, so that the same 

 kinds may be found in widely separate waters. Let us 

 see them in the halo of Hudson's eulogy : " To gaze into 

 that wonderful world which lies in a drop of water, crossed 

 by some atoms of green weed ; to see transparent living 

 mechanism at work, and to gain some idea of its modes of 

 action ; to watch a tiny speck that can sail through the 

 prick of a needle's point ; to see its crystal armour flashing 

 vdth ever-varying tints, its head glorious with the halo of its 

 quivering cilia ; to see it gliding through the emerald 

 stems, hunting for its food, snatching at its prey, fleeing 

 from its enemy, chasing its mate (the fiercest of our 

 passions blazing in an invisible speck) ; to see it whirling 

 in a mad dance to the sound of its own music — the music of 

 its happiness, the exquisite happiness of living, — can any 

 one who has once enjoyed this sight, ever turn from it to 

 mere books and drawings, without the sense that he has 

 left all fairj'land behind him?" Not less lively than the 

 Rotifers are crowds of minute crustaceans or water-fleas 

 which row swiftly through the clear water, and are eaten in 

 hundreds by the fishes. But there are higher forms still : 

 crayfish, and the larvae of mayflies and dragonflies, mussels 



