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. 



FIG. 227. Map showing the spread of the Canadian Pondweed (Elodea 

 canadensis} after its appearance in England in 1847. The .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 by the Prickly Pear in Australia. In 



