174 GENERAL PRINCIPLES OF ZOOLOGY. 



V. GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



The Different Faunal Regions. Even a superficial knowledge 

 of the mode of distribution of animals shows that the animal fauna 

 in different regions of the earth has an essentially different char- 

 acter. In part this difference of fauna is the immediate result of 

 climatic differences. The polar bear, arctic fox, eider-ducks, and 

 many aquatic birds are restricted to the polar zones, because they 

 cannot endure more than a certain degree of warmth; on the other 

 hand, the larger species of cats, the apes, the humming-birds, etc., 

 occur only in tropical or sub-tropical regions, because they are not 

 sufficiently protected against cooler weather. 



Climate not the Only Factor. If climate were the sole factor 

 determining distribution, the faunal character of two lands which 

 have similar climatic conditions would be essentially the same; 

 conversely, the separate regions within a continuous territory 

 extending through several climatic zones must have quite different 

 faunas, according as they are nearer the equator or the poles. But 

 such is not the fact; two tropical countries may differ more widely 

 in the characteristics of their fauna than the hot and cold regions 

 of one and the same country. 



Factors in Distribution. Modern zoology endeavors to explain 

 these peculiar conditions by regarding the present distribution of 

 animals as the product of two factors: the gradual changes of the 

 animal world, and further the gradual changes of the earth's sur- 

 face on which the animals are distributed. The history of the 

 earth as disclosed by geology shows two facts: (1) that the con- 

 nexions between parts of the earth have varied greatly; that, for 

 example, at a time when the Mediterranean had not yet reached 

 its present extent, Morocco, Algiers, Tunis, and Egypt were more 

 closely united with the European coast of the Mediterranean than 

 with the southern part of the African continent separated from 

 them by the Sahara; (2) that considerable variations of climate 

 have taken place : there prevailed in Europe in the tertiary period 

 a subtropical climate which rendered possible the existence of 

 animals which now occur in Algeria (lions). But later a glacial 

 period began, which introduced over a wide area of the European 

 continent the conditions of arctic life, and consequently a fauna 

 of northern animals (reindeer). Hand in hand with the geological 

 changes went changes in the animal world, the then existing 

 species dying out under the change of conditions, or forming new 



