INTRODUCTION 147 



is not a true habitat — it is rather one of the ecological factors in the 

 life of land animals. Its low density makes it impossible for living 

 organisms or their developmental stages to float in it continuously, and 

 air-breathing animals are therefore superficially distributed and in 

 principle confined to the earth's surface. In the ocean, living organisms 

 are permanently suspended, and may be represented in all their stages 

 at all depths. It is true that the strata are not equally dense in popula- 

 tion; the upper ones have in general the larger number of inhabitants. 

 Very few animals are found in the deeper regions, but living animals 

 have been brought to the surface from even the greatest depths. 



Terrestrial life occupies but a thin stratum which, even in forests, 

 does not ordinarily exceed some 30 m. in thickness. When this is con- 

 trasted with the mean depth of the oceans of almost 4000 m., and when 

 we remember that the ocean occupies more than twice the surface area 

 of the land, it is readily seen that there is over 300 times the space 

 available for marine as contrasted with terrestrial organisms. 



In spite of the much greater extent of their domain, the number of 

 species of the inhabitants of the sea is much smaller than that of the 

 air-breathers. On the basis of Pratt's figures of 1935, out of 822,000 

 species something like one-fifth are aquatic animals, including the 

 fresh-water forms. From this it may be concluded that the evolution of 

 species is less favored in the sea. This has its reason in the great uni- 

 formity of habitat conditions, and the much smaller development of 

 barriers to distribution. The isolation of a group of animals, which 

 contributes so much to its evolution into new forms, is made difficult 

 by the confluence of all parts of the ocean, while on land such isolation 

 is favored. 



Despite the smaller number of species in so much greater a space, 

 the animal life of the sea exhibits a greater diversity of form than does 

 that of land and fresh water; the variety of structure among marine 

 animals is much greater. Only 3 classes out of 47 recognized by Parker 

 and Haswell, the Onychophora, Myriapoda, and Amphibia, are wholly 

 wanting in the sea. On the other hand, 19 classes, including the entire 

 phylum Echinodermata, are purely marine. When a phylum has repre- 

 sentatives in fresh water and on land as well as in the sea, the former 

 are less varied in form than their marine relatives: thus, for example, 

 the Mollusca, with only the Pulmonata and a few Prosobranchiata on 

 land as compared with the varied marine classes ; or the Chordata with 

 the exclusively marine subphyla Acrania, Enteropneusta, and Tunicata, 

 and the predominantly marine Cyclostomata, Selachia, and Pisces, as 

 compared with the four air-breathing classes. Among the Arthropoda, 



