vi THE SPKEADING OF FOOD-PLANTS 73 



hedges, are not suitable for animals' food, but forms 

 almost without spines occur in the Mediterranean 

 region. This is a very curious fact which it is 

 worth taking note of. The prickly pear was not 

 originally a native of the Mediterranean region, but 

 in that region enormous areas of land occur which 



FIG. 17. Prickly Pear, near Dordrecht, South Africa. 



are very well suited to its needs. Once intro- 

 duced, then, it flourished apace, ran wild, and spread 

 over great areas of the land. This is not at all 

 uncommon with introduced plants and animals. 

 We all know how rabbits have spread over New 

 Zealand and Australia, how some of our weeds, 

 such as thistles, introduced accidentally, have done 

 the same thing. In short, it is quite common to 



