north to 40 deg. 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.
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