28 
DARWINISM 
CHAP. 
of America where open pastures offered suitable conditions. 
Asses, about fifty years after their introduction, ran wild and 
multiplied so amazingly in Quito, that the Spanish traveller 
Ulloa 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 upon 
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 twenty 
months 25,000 wild pigs. 1 Now, 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 now 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 weeds 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 
1 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, sheep-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. 
