12 MANUAL OF ZOOLOGY 



are to be distinguished. There are animals such as jelly- 

 fishes, which float on or near the surface of the water, and 

 are carried about passively by currents ; such forms are 

 included under the term Plankton. Most fishes, whales, and 

 cuttle-fishes, on the other hand, are strong swimmers, and 

 are able to traverse the water at will in any direction ; they 

 together constitute the Nekton. Finally, such animals as 

 crabs, oysters, sponges, zoophytes, etc., remain permanently 

 fixed to or creep over the surface of the bottom, and are 

 grouped together as the Benthos. 



Under the head of geographical distribution we have such 

 facts as the absence of all land-mammals, except bats in 

 New Zealand and the Polynesian Islands, the presence of 

 pouched Mammals, such as kangaroos and opossums, only 

 in some parts of America and in Australia and the adjacent 

 islands, the entire absence of finches in Australasia, and so 

 on. We find, in fact, that the fauna — i.e. the total animal 

 inhabitants — of a country is to a large extent independent 

 of climate, and that the fauna? of adjacent countries often 

 differ widely. In fact, it is convenient in studying the geo- 

 graphical distribution of animals largely to ignore the ordi- 

 nary division into continents, and to divide the land-surface 

 of the globe into what are called zoo-geographical regions. 



There are still two departments of zoological science to 

 be mentioned. As it is impossible to have a right under- 

 standing of a machine without knowing something of the 

 purpose it is intended to serve, so the morphological study 

 of an animal is imperfect without some knowledge of its 

 Physiology, i.e. of the functions performed by its various 

 parts, and the way in which they work together for the 

 welfare of the whole. 



Not only may we study the action of a given animal's 

 organs, but also the actions of the animal as a whole, its 



