48 THE THEORIES OF EVOLUTION 



factor did not destroy a large number of individ- 

 uals. 



When our domestic animals run wild and circum- 

 stances are favourable to them, as in the case of cat- \ 

 tie and horses in South America and Australia, their 

 increase is astonishingly rapid. So it is with plants. 

 Cases could be cited of plants newly introduced which J 

 haW become common throughout whole islands in a 

 period of less than ten years. Several of the plants 

 which are now commonest over the wide plains of La 

 Plata, clothing square leagues of surface, have been 

 introduced from Europe. The elephant is reckoned 

 the slowest breeder of all known animals. Yet if 

 one elephant brings forth six young, then after a 

 period of 750 years, there would be nearly nineteen 

 million elephants alive, all descended from the first 

 pair. If all species of animals increased at the same 

 ratio, their numbers would become so inordinately 

 great that no country could support them. 1 



The increase of living things, however, is not so 

 easy by any means; their existence is dependent on a v 

 thousand conditions, on a thousand other living 

 things. Every one of us could cite examples off- 

 hand. Here are two offered by Darwin: several 

 hundred acres of a barren heath had been planted with 

 Scotch fir. The change in the native vegetation of 

 the planted part of the heath was most remarkable, 



i Origin of Species, passim. 



