DISTRIBUTION. 211 



regions within a continuous territory extending through 

 several climatic zones must have quite different fauna, 

 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 pres- 

 ent 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 surface, serving for the ex- 

 tension of the animal kingdom. The history of the earth 

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

 nections 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 

 there took place also changes in the animal world ; the 

 then present species dying out under the change of condi- 

 tions of existence, or forming new species through gradual 

 variations. Thus animal geography constitutes an extremely 

 complicated problem, the solution of which necessitates a 

 comprehensive course of preliminary work. It must be 

 known with certainty how the connections between the 

 continents, and the distribution of climates, have changed, 

 particularly in the later geological periods; further, we 

 must study, not only how animals at the present time dis- 

 tribute themselves over the earth's surface, but also how 



