412 DISTRIBUTION IN SPACE AND TIME 



in widely distant parts of the world have left descendants, while 

 all the marine types were doomed to extinction. 



Dispersal of Animals. — A species which meets with any 

 success in the struggle for existence increases largely in numbers 

 and, led chiefly by the search for food, comes to occupy a tract 

 of land or sea of continually increasing size {i.e. it widens its 

 area of distribution) until prevented by natural causes from migrat- 

 ing farther. The means of locomotion possessed by such a 

 species necessarily plays an important part in the matter. The 

 power of flight, for example, often renders wide dispersal possible, 

 as in the case of Bats. But there are usually certain physical 

 barriers which put a stop to the migratory movements of most 

 kinds of animal. Mammals other than Bats are unable to cross 

 even narrow arms of the sea, while mountain chains and deserts 

 often prove potent checks to further advance. And for every 

 other group of land forms obstacles of varying kind present 

 themselves. Even in the case of marine species limits are im- 

 posed by temperature, depth of water, supply of suitable food, 

 competition with other species, and so on. 



ZooGEOGRAPHiCAL REGIONS OF THE Land. — It is generally 

 considered that Mammals afford the best means of dividing the 

 land into regions possessing characteristic faunas, and as the 

 areas thus demarcated answer fairly well for Birds, the sub- 

 divisions made by W. L. Sclater on this basis, and afterwards 

 adopted by Wallace, will here be given. A reference to the 

 accompanying map (fig. 1286) will show that the boundaries 

 between Sclater's six great regions are largely constituted by 

 physical barriers. Each of these primary subdivisions is again 

 divided into sub-regions. All that can be attempted here is a 

 brief account of the leading features of the large distributional 

 areas, especially with reference to Mammals and Birds. So many 

 incorrect ideas are current, even among educated persons, about 

 the distribution of well-known animals, that no attempt will be 

 made to avoid details that will be commonplace to some readers. 

 Most of the forms of life mentioned in this chapter will be found 

 to have received notice in other connections. It may be well 

 first of all to enumerate the regions and sub-regions. 



I. Pal.earctic Region. — Europe, — all but the south of Asia, 

 — and Africa north of the Sahara. Sub- Regions: — i. European; 

 2. Mediterranean; 3. Siberian; 4. Manchurian. 



