28 DARWINISM chap. 



of America where open pastures offered suitaHe conditions. 

 Asses, about fifty years after their introduction, ran wild and 

 multipHed so amazingly in Quito, that the Spanish traveller 

 Uiloa describes them as being a nuisance. They grazed 

 together in great herds, defending themselves with their 

 mouths, and if a horse strayed among them they all fell xipon 

 him and did not cease biting and kicking till they left him 

 dead. Hogs were turned out in St. Domingo by Columbus 

 in 1493, and the Spaniards took them to other places where 

 they settled, the result being, that in about half a century 

 these animals were found in great numbers- over a large part 

 of America, from 25° north to 40° south latitude. More 

 recently, in New Zealand, pigs have multiplied so greatly in 

 a wild state as to be a serious- nuisance and injury to 

 agriculture. To give some idea of their numbers, it is stated 

 that in the province of Nelson there were killed in tAventy 

 months 25,000 wild pigs.^ Noav, in the case of all these animals, 

 we know that in their native countries, and even in America 

 at the present time, they do not increase at all in numbers ; 

 therefore the whole normal increase must be kept down, 

 year by year, by natural or artificial means of destruction. 



Rapid Increase and Wide Spread of Plants. 



In the case of plants, the power of increase is even greater 

 and its effects more distinctly visible. Hundreds of square 

 miles of the plains of La Plata are noAv covered with two or 

 three species of European thistle, often to the exclusion of 

 almost every other plant ; but in the native countries of these 

 thistles they occupy, except in cultivated or waste ground, a 

 very subordinate part in the vegetation. Some American 

 plants, like the cotton-weed (Asclepias curassavica), have now 

 become common Aveeds over a large portion of the tropics. 

 White clover (Trifolium repens) spreads over all the temperate 

 regions of the world, and in New Zealand is exterminating 

 many native species, including even the native flax (Phormium 



^ Still more remarkable is the increase of rabbits both in New Zealand and 

 Australia. No less than seven millions of rabbit-skins have been exported 

 from the former country in a single year, their value being £67,000. In both 

 countries, sheeii-runs have been greatly deteriorated in value by the abundance 

 of rabbits, which destroy the herbage ; and in some cases they have had to be 

 abandoned altogether. 



