PRINCIPLES OF DISTRIBUTION. 229 



theories have been advanced to account for secular changes of climate. The most satis- 

 factory is one by Wallace which attributes them to geographical revolutions (previously 

 suggested by Lyell), influenced by astronomical changes (variations in excentricity of 

 earth's orbit and movements of precession, as advanced by Croll). 



All the preceding changes must have exerted a profound influence upon organisms, 

 and throw light upon many problems of distribution. 



Areas of Distribution. May be mapped out for species genera, families, and orders. 

 In all cases : (1) Size and nature of boundaries very variable. (2) Need not be continuous. 



(a.) Exs. of Limited Areas. The marmot only found in the Alps. A species of 

 humming-bird confined to the crater of the extinct volcano Chiriqui in Veragua. 



Six genera of Lemurs are peculiar to Madagascar. The family Galeopithecidce (including 

 the single genus Galeopithecus) is limited to Malacca, Sumatra, Borneo, and the 

 Philippines. The order Monotremata only occurs in Australia, Tasmania, and New 

 Guinea. 



(6.), Exs. of Extensive Areas. The leopard is distributed through the whole of Africa 

 and S. Asia to Borneo and E. China. The genus Felis (cat, lion, leopard, &c.) 

 ranges over most of the globe except Australia, the Pacific Islands, W. Indies, 

 Madagascar, and the more northerly parts of X. America and Asia. The family 

 Vespertilionidce, including 200 species of small insect-eating bats, occurs everywhere 

 within the tropical and temperate zones; while the family M uridce (rats, mice, &c.) is only 

 absent from Polynesia and New Zealand. 2. Discontinuity is generally a sign of antiquity, 

 the two or more parts being remains of a once continuous distributional area, in part of 

 which extinction has occurred. Changes in the distribution of land and sea have broken 

 up many once continuous areas. Examples. The variable hare (Lepus variabilis), 

 Europe and Asia N. of 55 ; Alps, Pyrenees, and Caucasus. The genus Tapirus, 

 S. America, S. E. Asia. Centetidce (a family of the Insectivora), Madagascar, Cuba, 

 Hayti. 



Ganoid fishes are now represented by genera with the 'following distribution: 

 Acipenser, N. temperate and Arctic regions. Most species marine, others are found in 

 the Caspian Sea, Black Sea, and N. American lakes, with their rivers, also in the 

 Danube, Mississippi, and Columbia. Scapbirhynchus, Mississippi and tributaries. 

 Polyodon, Mississippi, and Yang-tse-Kiang. Polypterus, Nile and W. African rivers. 

 Calamoichtbys, rivers of Old Calabar. Amia, fresh-water, United States. Lepidos- 

 teus, fresh-water, N. America to Mexico and Cuba. Ganoid fishes are of great 

 geological antiquity, and were formerly a widely spread marine group. Most of the forms 

 now surviving have gradually accommodated themselves to a life in rivers, lakes, &c., where 

 the struggle for existence is less severe. 



The peculiar distribution of the Dipnoi (p. 161) can be explained similarly. 



The Marsupialia and Edentata (p. 235) are also good examples of interrupted areas of 

 distribution. 



Zoological Regions, characterised by the presence of peculiar families and genera, 

 and by fne absence of other families and genera, have been formed for sea and land. The 

 most useful division of the latter is chiefly based on the Mammalia, but applies very well 

 to birds and reptiles and fairly to other groups. The regions thus established are six in 

 number. 



I. Palcearctic Region. Temperate Europe and Asia and N. temperate Africa. Extends 

 W. to Iceland, the Azores, and Cape Verde Islands, and E. to Behring Straits and 

 Japan. Southern boundary somewhat indefinite, tropic of Cancer in Africa and Arabia, 

 river Indus, Himalayas, Nanling mountains. 



II. Ethiopian Region. Africa and Arabia S. of the tropic of Cancer, and including 

 Madagascar. 



III. Oriental Region. Asia, S. of Region I., and the western part of the E. Indies. 

 The E. boundary of this region (Wallace's line) passes between Bali, Borneo, and the 

 Philippines on the one hand, Lombok and Celebes on the other. The former islands are 

 therefore in the Oriental Region, the latter in the Australian Region. 



IV. Australian Region. Australia, New Guinea, New Zealand, with the smaller 

 islands from Wallace's line to the Marquesas and Low Archipelago, and the tropic of 

 Cancer to the Macqnarie Islands. 



V. Neotropical Region. S. America, the W. Indies, and tropical N. America, with the 

 exception of the central part of the Mexican table-land. 



VI. Nearctic Region. Arctic and temperate N. America, with the central part of the 

 Mexican table-land. 



