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Show 16 DARWINISM. CHAP. wild in New Zealand. But Sir Joseph Hooker in~orms us that the late Mr. Bidwell habitually scattered Australian seeds during his extensive travels in New Zealand, .yet only two or three Australian plants appear t~ have. established themselves in that country, and these only m cultivated or newly moved soil. These few illustrations sufficiently show that all the plants of a country are, as De Candolle says, at war with each oth~r, each one struggling to occupy ground at t~~ expense ?f Its neighbour. But, besides this direct competitJOn, there Is one not less powerful arising from the exposure of almost all pl.ants to destruction by animals. The buds are des.troyecl by. b1rds, the leaves by caterpillars, the seeds by weeVIls; some msects bore into the trunk, others burrow in the twigs and leave ; slucrs devour the young seedlings and the tender shoots, wirewo~ ms gnaw the roots. Herbivorous mammals devour m~ny species bodily, while some uproot and devour the buned tubers. In animals, it is the eggs or the very young that suffer most from their various enemies; in plants, the tender seedlings when they first appear above the ground. To illustrate this latter point Mr. Darwin cleared and dug a piece of ground three feet long and two feet wide, and then marked all the seedlings of weeds and other plants which came up, noting what became of them. The total number was 357, and. out of these no less than 295 were destroyed by slugs ::mel insects. The direct strife of plant with plant is almost equnJly fatal when the stronger are allowed to smother the weaker. When turf is mown or closely browsed by animals, a number of strong and weak plants live together, because none are allowed to grow much beyond the rest ; but Mr. Darwin found that when the plants which compose such turf are allowed to grow up freely, the stronger kill the weaker. In a plot of turf three feet by four, twenty distinct species of plants were found to be growing, and no less than nine of these perished altogether when the other species were allowed to grow up to their full size.1 But besides having to protect themselves against competing plants and against destructive animals, there is a yet deadlier 1 The Origin of Species, p. 53. II TilE 'TRUGGLE FOR EXIS'l'Kr CE 17 enemy in tho f_orces of inorganic nature. Each species can susta~n a cCL·tam amo~mt of heat and cold, each requires a certam amount of m01 sture at the right season, each wants a pr?per amount of light or of direct sunshine, each needs ~ertam e~ements. in the. s?il ; the failure of a due proportion m these morgamc conchtwns causes weakness, and thus leads to speedy death. ~he struggle for existence in plants is, therefore, thro~fold m. char~cter ~nd infinite in complexity, and the result Is seen m the1r cnnonsly irregular distribution o~er: the face of the earth. Not only has each country its d1stmct plants, b~1t every valley, every hillside, almost every h~d~erow, has a d1fferen~ set of plants from its adjacent valley, hills~de, or hedgerow- If not always different in the actual spe~1es yet ve~'Y different in comparative abundance, some ~h1eh are rare m the one being common in the other. Hence It happens th~t slight changes of conditions often produce great -changes m the flora of a country. Thus in 17 40 and the t"':o following years the larva of a moth (Phalmna, graminis) committed such destruction in many of the meadows of Sweden that the gras.s was greatly climini bed. in quuntity, and many plants which were before choked by the grass sprang up, and the wound became variegated with a multi~ ude of ~Ifferent specws of flowers. The introduction of goats mto the Island of t. Helena led to the entire destruction of the native forests, consisting of about a hundred distinct species of trees and shrubs, the young plants being devoured by the goats as fast as they g:ew up. The camel is a still greater ene.rny to woody vegetatiOn than the goat, and Mr. Mar. h believes that forests would soon cover considerable tracts of the Arabian and African deserts if the goat and the camel were remove 1 from them. I Even in many parts of our own country the exi. ten ce of trees is dependent on the absence of cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in Surrey, a few clumps of old Scotch firs, but no young trees over hundreds of acres. Some portions of the heath had, however, been enclosed a few years before, and these enclosures were crowded with young fir-trees growing too closo toge~her for. all to live; and these were not sown or planted, nothmg having been done to the ground beyond enclosing it 1 The Earth as Modified by Human Action, p. 51. c |