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. 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 tenax), a large plant with iris-like leaves 5
or 6 feet high. Mr. W.L. Travers has paid much attention to the effects
of introduced plants in New Zealand, and notes the following species as
being especially remarkable. The common knotgrass (Polygonum aviculare)
grows most luxuriantly, single plants covering a space 4 or 5 feet in
diameter, and sending their roots 3 or 4 feet deep. A large sub-aquatic
dock (Rumex obtusifolius) abounds in every river-bed, even far up among
the mountains. The common sow-thistle (Sonchus oleraceus) grows all over
the country up to an elevation of 6000 feet. The water-cress (Nasturtium
officinale) grows with amazing vigour in many of the rivers, forming
stems 12 feet long and 3/4 inch in diameter, and completely choking them
up. It cost L300 a year to keep the Avon at Christchurch free from it.
The sorrel (Rumex acetosella) covers hundreds of acres with a sheet of
red. It forms a dense mat, exterminating other plants, and preventing
cultivation. It can, however, be itself exterminated by sowing the
ground with red clover, which will also vanquish the Polygonum
aviculare. The most noxious weed in New Zealand appears, however, to be
the Hypochaeris radicata, a coarse yellow-flowered composite not
uncommon in our meadows and waste places. This has been introduced with
grass seeds from England, and is very destructive. It is stated that
excellent pasture was in three years destroyed by this weed, which
absolutely displaced every other plant on the ground. It grows in every
kind of soil, and is said even to drive out the white clover, which is