THE DISTRIBUTION OF SPECIES. 219 



with time. They change with space because, as they 

 move over the globe, with distance comes barriers, with 

 barriers isolation, and any degree of isolation brings 

 some change of conditions. This means, sooner or later, 

 a distinction of species. * 



Species change with time, because time brings changes 

 in conditions. Epoch-making events of one sort or an- 

 other come in to break even the most 

 How species monotonous existence. If time could 



change with ^^^ ^^ evenly there need arise no 



time. , . ,.. , . , . 



change m life, adaptation being once 



established. If space were absolutely uniform without 

 barriers or variation of conditions, life would flow on 

 as uniformly. Where there is most monotony in condi- 

 tions, as in the depths of the sea, there is least change 

 in life, least formation of new species, and least tendency 

 to progress through natural selection. 



Similar to geographical isolation in its nature and 

 effects is physiological isolation. This appears in the de- 

 velopment, in isolated races, of antipa- 

 Physiological ^^-^^^ which serve as barriers to prevent 

 isolation. , • , j- -^i n- j 



the interbreeding with allied races or 



species. This condition among animals is the homo- 

 logue of race hostility among men. Such a feeling of 

 mutual aversion, whether accompanied by anatomical 

 distinctions or not, must be at times a strong factor in 

 the differentiation of species. 



The study of the problems of geographical distribu- 

 tion is possible only on the theory of the derivation of 



species. If we view all animals and 

 Meaning of facts j^^^^ ^^ ^^^ ^.^^^j^^ ^^ j^j creations 



of distribution. . . . , , , 



in the regions assigned to them, we have, 



instead of laws, only a jumble of arbitrary and meaning- 

 less facts. We have been too fully accustomed to the 

 recognition of law to believe that any facts are arbitrary 



