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130 THE THEORY OF EVOLUTION 



extended their range to Argentina. The explanation 

 of the discontinuity is the same as in the tapirs, the 

 former occupation of the intervening lands by the 

 family in question and their eventual extinction in 

 those lands. 



iird case is rather different. On the tops of the 

 White Mountains in New Hampshire grows an 

 assemblage of plants, to find which in the lowlands, 

 one must go hundreds of miles northward to Labra- 

 dor. The probable reason for this strange distribu- 

 tion is given by the glacial phenomena of the Pleis- 

 tocene; when the ice had buried the northern part of 

 the continent, Arctic animals and plants were driven 

 far to the South, the walrus coming down to the 

 coast of Georgia and the musk-ox to Kentucky and 

 Oklahoma. With the amelioration of the climate and 

 retreat of the ice, the Arctic plants and animals once 

 more went northward and ascended the mountains, 

 as these were freed from the ice. When present 

 climatic conditions had been attained, the Arctic 

 forms had disappeared from the foot of the moun- 

 tains, but remained on their cold summits as colonies 

 of the great Arctic invasion. The same thing hap- 

 pened in Europe, where Arctic plants still flourish in 

 the high Alps. Thus, in all cases where the history 

 of the animals or plants involved can be obtained, 

 the explanation of discontinuous distribution is the 

 same, former occupation of the intervening area. 

 The striking contrast between North and South 

 America is expressed in the scheme of classification; 



