OCR Text |
Show 28 DARWINISM OHAP. of America where open pastures offered su~table con~11~ion\ Asses, about fifty years after their introdhnct1Son, r.a1h1 w1 ( a 11 1u multiplied so amazingly in Quito, that. t e pams trave er Ulloa describes them as being a nmsance. The! grazc.<l together in great herds, defending themselves w1th the1r ths and if a horse strayed among them they all fell upon hmiomu an'd did not cease biting and kicln·n g. ti'1 1 t h ey 1e f t 1m · n dead. Hogs were turned out in St. Dommgo by Colnmhm; in 1493, and the Spaniards took the~ to other places where they settled, the result b~ing, that m about half a ccntur! these animals were found m great numbers ove~ a large p:u t of America from 25° north to 40° south la,t1tmle. More recently, in' New Zealand, pigs ~ave m~ltiplied so $r?:~tly in a wild state as to be a senous nmsance and Hl]tlry to aariculture. To <rive some idea of their numbers, it is stated that in the provft1ce of Nelson there were killecl 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 Amcri c:L at the present time, they do not increase at all in nnmherR; therefore the whole normal increase must be kept clown, 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 gr oa,tor and its effects more distinctly visible. Hundreds of . <ptarc miles of the plains of La Plata are now covered with two or three species of European thistle, often to tho exclusion of almost every other plant; but in the native countries of these thistles they occupy, except in cultivated or wa. to <rronnd, :1 very subordinate part in tho vegetation. Some American plants, like the cotton-weed (Asclepias curas n,vi ca,), have now hocome common weeds over a large portion of the tropie:-:. White clover (Trifolium repens) spreads over all tho tomperat<' regions of the world, and in Now Zealand is oxtermin n:ing many native species, including even the native flax (Phormnnn I Still more remarkable is i11e increase of rabhiis hotl1 in New Zeal:wcl a11tl Australia. No le. s thau seven millions of rabhit-skius l1av' been x por l<·t l fron1 the former country in a single year, their value heing £67,000. fn holl1 counLrie~, sheep-runs have been greatly deteriorn.t •d in value hy t he ah nn clancc of rabuit.·, which destroy the herhagc; and in some cases Lhcy have had to he auanuoneu altogether. II THE STRUGGLE FOR EXISTENCE 29 tonax), a large plant wit~ iris-like leav?s 5 or 6 feet high. Mr. W. L. Travers has pmd much attentwn to tho ofT'ects of introduced plants in New Zealand, and notes the following species as being especially remarkable. The common knotgrass (Poly~onum aviculare) grows m?st l.uxuriantly, single plants covermg a space 4 or 5 feet m dmmoter, 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 (Sonc~us oleraceus) grows all over tho country up to an elevatw~ of 6000 .feet .. The ':ator-cress (Nasturtium officinalo) grows With amazmg VIgour m many of the rivers, forminrr stems 12 feet long and ! inch in diameter, and complete]; choking them up. It cost £300 a year to keep the A von 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 preventincr cultivation. It can, however, be itself exterminated by so~ina the ground with ;ed clover, which will also vanquish b the Polygonum avJCulare. The most noxious weed in New Zealand appears, however, to be the Hypochreris 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 throe 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 usu'ally so powerful in taking possession of the soil. In Australia another composite plant, called there the Capowe~ d (Cryptostemma calendulaceum ), did much damage, and was noticed by Baron Von Hugel in 1 33 as "an unexterminable weed"; but, after forty years' occupation, it was found to give way to the dense herbage formed by lucerne and choice grasses. . In Ceylon we are told by Mr. Thwaites, in his Enwnem~ wn of Ceylon Plants, that a piant introduced into the Island less than fifty years ago is holpina to alter 1 he ch~ra~ter of the vegetation up to an olevati~n of 3000 fo t. Th1s 1s the Lantana mixta, a verbenaceous plant introduced |