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 



Ireland 1836 



I860 

 Germany 1565-6? 



K.J.S. 



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

 i canadensis) after its appearance in England in 1847. Tne .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 la.nd, is presented by the Prickly Pear in Australia. In 



