DISTRIBUTION 161 



regions of the earth has an essentially different character. In part this is the 

 immediate result of climatic differences. The polar bear, arctic fox, eider- 

 ducks, 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 warmer regions, because they are 

 not sufficiently protected against cooler weather. 



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 of a continuous territory extending 

 through several climatic zones must have 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 their fauna than tlie hot and cold regions of one and 

 the same country. 



Factors in Distribution. — ^lodern zoology endeavors to explain these 

 conditions by regarding the present chstribution of animals as the product of two 

 factors: the gradual changes of the animal world, and the gradual changes of the 

 earth's surface on which the animals are distributed (p. ;};}). The history of 

 the earth as disclosed by geology shows two facts: (i) that the connections 

 between parts of the earth have varied greatly; that, for example, at a time when 

 the ^Mediterranean had not yet readied 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 large part 

 of Europe arctic conditions, and consequently a fauna of northern animals 

 (reindeer) which has left a few traces (Alpine hare) in the glacier regions. Hand 

 in hand with the geological changes went changes in the animal world, tlie then 

 existing species dying out under the change of conditions, or perhaps forming 

 new species through gradual variations. Thus the distribution of animals 

 constitutes an extremely complicated problem, for tire solution of which it must be 

 known how the climates and the connections between the continents have 

 changed, particularly in the later geological periods; further, not only how 

 animals are distributed at present over the earth's surface but also how they were 

 distributed in earlier times. Finally, we must have clear and detailed ideas of 

 the relationships and interrelationships of animals. 



It will be a long task to solve all the problems. What has been accomplished 

 so far can only be regarded as showing tliat zoology, with its prevailing ^■iews 

 of the changes of animals and of the earth, is on the right track. Two regions, 

 separated early in the earth's history and never again connected, must have 

 greater differences in faunal characters than two lands still connected or only 

 recently separated. We travel in the northern hemisphere and find in widely 

 separated regions strikingly similar faunae, wliile under the equator or in the 

 southern hemisphere, under the same conditions for each region, striking 

 differences are seen. This is explained on the hypothesis that in all past periods 

 as now the land masses of the northern hemisphere have been closely connected, 

 while the parts of the continents extending to the south have been separated 

 through most of the earth's history. 



Students of distribution have attempted to define the great faunal areas of 

 the earth,, the faunal provinces or regions, and within these the subregions. 

 These provinces have been based chiefly upon the distribution of mammals, 

 less upon that of birds and other animals; for the distribution of mammals is 

 chiefly determined by those changes of the earth's surface which are best known 

 geologically and possess most interest. Elevation or depression of the earth's 

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