CHAPTER XXVIII. 



GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



As similar animals tend to occur where the conditions of 

 life are similar, we are warranted in speaking of a pelagic 

 fauna, an abyssal fauna, a littoral fauna, and so on. Let us 

 briefly consider this grouping of animals according to their 

 haunts. 



Pelagic. The pelagic fauna includes all the animals of 

 the open sea, both drifters (Plankton) and swimmers 

 (Nekton). The physical conditions in which they live are 

 very favourable, there is room for all, sunshine without 

 risk of drought, and an evener life throughout the day and 

 throughout the year than is to be found elsewhere except in 

 the abysses of the deep sea. Moreover, the minute pelagic 

 Algae afford an inexhaustible food-supply to the animals. It 

 is not surprising, therefore, to find that the open sea has 

 been peopled from the earliest times of which the rocks give 

 us any life record. 



The fauna is representative, exhibiting great variety of 

 types, from the minute Noctiluca which sets the waves 

 aflame in the short summer darkness, to the giants of 

 modern times the whales. It includes a few genera of 

 Foraminifera, rich in species, most Radiolarians, many 

 Infusorians, Medusae and Medusoids, Siphonophora and 

 Ctenophora, many "worms" and two Holothurians, a 

 legion of Crustaceans and a few Insects (Halobatidae), such 

 Molluscs as Pteropods, Heteropods, and many of the 

 Cephalopods, such Tunicates as Salpa and Pyrosoma, many 

 fishes, a few turtles and snakes, besides some well-known 

 birds and mammals. 



The fauna of the open sea is representative, but there are 

 few of the types which we can suppose to have lived there 



