GEOGRAPHICAL DISTRIBUTION. 115 



is less behind the Old World than is Australia, 

 but even in America multitudes of European 

 animals and plants have established a firm 

 footing, while a very much smaller number 

 of American species have become naturalised 

 in Europe.* 



The American water-weed, one of the few 

 American plants which have become naturalised 

 in England, is an exception which proves the 

 rule ; for it is a water-plant, and the rivers of 

 Europe cannot compare with those of America 

 in size or importance. It is therefore not 

 surprising that a plant which has been trained 

 to the severe competition of river-life in 

 America, should be able to hold its own in 

 the small rivers and canals of Central Europe. 



It occasionally happens that the same genus, 

 or even the same species, is met with not in ad- 

 jacent districts, but in widely separated regions. 

 Mivart lays great stress upon these cases,f and 

 appears to think that the same species may 

 sometimes be developed in different countries. 



* Compare an interesting article on Imported Insects, and North 

 American Insects in Riley's " Second Report on the Insects of Missouri," 

 pp. 8-15. 



t " Genesis of Species,"ch. 7 



I 2 



