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.[10] 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 cuiussayica), have now become common weeds over a large portion of the tropics.