1 66 EVOLUTION OF TO-DAY. 



extended the range of the Colorado potato-beetle 

 from small localities of the West over almost the 

 whole of our country, and even into Europe. The 

 presence of the tsetze-fly in Africa makes large terri- 

 tories uninhabitable by cattle, which are found both 

 north and south of the infested regions. Many 

 plants depend upon insects who carry their pollen 

 from one to the other. The insects are dependent 

 on the insectivorous birds, and these upon the 

 abundance of bird-eating mammals. When we con- 

 sider the multitudes of such modifying conditions 

 which are known, it becomes evident that there 

 must be even greater numbers unknown and un- 

 thought of. The accidental introduction of new 

 animals into an old country ; the slightest changes 

 in climate; the changes in the course of river-beds; 

 the disappearance or growth of forests; the abun- 

 dance or absence of certain kinds of food ; changes 

 in the level of the land ; the removal of slight bar- 

 riers which have prevented free migration between 

 contiguous localities; all of these circumstances 

 produce profound modifications in the inhabitants 

 of any country, and teach us that we must not be 

 surprised if it is frequently impossible to say why 

 any animal is found in one locality and not in the 

 neighboring ones. Moreover, the present distribu- 

 tion must be dependent on the past, and we know 

 too little of past history to give any hope of a com- 

 plete explanation. 



Animals not Distributed According to Climate. 



It might have been supposed, indeed would nat- 

 urally follow from the standpoint of special crea- 



