DEEP-SEA LIFE. 75 



" sea-serpents," some turtles, many Birds, and a few Mam- 

 mals, such as the Polar bear. 



(3.) Deep-Sea Life. — From the shore contingents have 

 doubtless gone out to the open sea whence their ancestors 

 came, but others have probably followed outward-swept 

 food down to the great depths. This is the most probable 

 origin of the deep-sea fauna, which includes Protozoa, 

 numerous horny but especially flinty Sponges, a few corals, 

 "worms," all kinds of Echinoderms in abundance, some 

 Molluscs and remarkable Fishes. That " animal life is pre- 

 sent on the bottom of the ocean at all depths," was one of 

 the important discoveries of the " Challenger" expedition 

 (1872-6), which, under Sir Wyville Thomson's leadership, 

 revealed a new zoological world. The scientific results, 

 worked out by numerous experts, and edited by Dr. John 

 Murray, form a monumental series of volumes, the com- 

 plete importance of which we have not yet had time to 

 appreciate, while the expedition was likewise successful in 

 prompting other nations to similar enterprises. 



" Depths beyond five hundred fathoms are inhabited 

 throughout the world by a fauna which presents generally 

 the same features throughout," for the conditions of life in 

 the great abysses are very uniform. It must be a weird 

 strange world of calm silence, without any light but gleams 

 of phosphorescence, and necessarily without any plants. 

 The inhabitants live on one another, but in great measure 

 on the Protozoa and other small creatures which sink from 

 the surface as they die. This food is probably very abun- 

 dant, and the struggle for subsistence proportionally slight. 

 The " ooze " which covers the floor of the sea, consists in 

 great part of the shells of Diatoms, Foraminifera, Radiolarians, 

 and Pteropods, which sink from the surface. At depths 

 between 400 and 2000 fathoms it usually consists of the 

 calcareous shells of Foraminifera, like those which form 

 our chalk clifis. This calcareous area is richest in animal 

 life. In greater depths, before reaching which calcareous 

 shells are dissolved, the flinty shells of Radiolarians abound ; 

 but in the greatest depths the shell-ooze is more or less 

 completely replaced by a material usually called " red-clay." 



(4.) Fresh-water Fauna. — Fresh water is the home of a 

 large number of Protozoa, a sponge {Spongilld), Hydra and 



