392 



GEOGRAPHICAL DISTRIBUTION 



fact is constantly being illustrated by the successful spread of 

 species after their first introduction into a country. For example, 

 the Canadian Pondweed {Elodea canadensis), which was brought 



K.J.S. 



BeXaiLim iHoXlani 



1560 

 &evmau>^ 1565-67 



Fig. 227. — Map showing tlic spread of the Canadian Pondweed {Elodea 

 canadensis) after its appearance in England in 1S47. Tlic broken 

 lines connect identical years. 



to England from America about 1847, now occupies almost every 

 waterway in this country (cf. Fig. 227). A similar phenomenon, 

 on land, is presented liy the Prickly Pear in Australia. In 



