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Show 22 DARWINISM CHAP. first place to the holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak, and beech appear to be the steps in the struggle for the survival of the fittest among the forest-trees of Denmark It may be added that in the time of the Romans th beech was the principal forest-tree of Denmark as it is now, while in the much earlier bronze age, represented by the later rema,ins found in the peat bogs, there were no beech-trees, or very few, the oak being the prevailing tree, while in the sti 11 earlier stone period the fir was the most abundant. The beech is a tree essentiaJly of the temperate zone, having its northern limit considern.bly southward of tho oak, fir, birch, or aspen, and its entrance into Denmark was no doubt dne to tho amelioration of the climate after tho glacial epoch had entirely passed away. We thus see how changes of climate, which are continually occurring owing either to cosmical or geographical causes, may initiate a struggle among plan ts which may continue for thousands of years, and which must profoundly modify the relations of the animal world, since the very existence of innumerable insects, and even of many birds and mammals, is dependent more or less completely on certain species of plants. The Struggle for Existence on the Pampas. Another illustration of tho struggle for existence, in which both plants and animals are implicateu, is afforded by the pampas of the southern part of South America. The absence of trees from these vast plains bas been imputed by Mr. Darwin to the supposed inability of the tropical and suhtropical forms of South America to thrive on them, and there being no other source from which they could obtain a supply; and that explanation was adopted by such eminent bot:tni ~b; as Mr. Bttll and Professor As.a Gray. This expbnatioH ha~ ~tlways seemed to me unsatisfactory, bccau. e there arc <tmple forests both in the temperate regions of the Anclos and on the w~wle west coast down to Terra del Fuego; and it is inconsistent w1th. what we know of the rapid variation and adaptation of spec10s t? new conditions. What seems a more satisfactory explanation has hcen given by Mr. Edwin Clark, a civil engineer, who resided nearly two years in the country and ll TilE STRUGGLE FOR EXI TENCE 23 paid ~uch attent~o~ to its natural history. He says : "The pecuhar charactcristlcs of these vast level plains which descend from the Anucs to the great river basin in unbroken monoto th b f . ny, are e a scnco o nvcrs or water-storage, and the periE>cli ·al occurrence .o~ droughts, . or 'siccos,' in the summer months. These conditJOns detcrmme the singular character both of its flora and fauna. "Tho soil is naturally fertlle and favourable for the rrrowth of trees, and they grow luxuriantly wherever they a~o pro~ ected. The euca~yptus is covering large tracts wherever it 1s enclosed, and willows, poplars, and the fi o· surround ever estancia when fenced in. 0 Y "The open plains are covered with droves of horses an.d. cattle, and overrun by numberless wild rodents, the ongmal tenants of the pampas. Dnrinrr tho lonrr periods of .drought, which arc so great a scourge t~ the country, the ·e ~mmals are sta1~ved by thousands, destroying, in their efforts to hve, every vestige of vegetation. In one of these ' siccos ' at the time of my visit, no less than 50,000 head of oxen 'and sheep and horses perished from starvation and thirst aftcrtearin<r deep out of tho soil every trace of vc(•·ctation i;lClndinn· th 0e . b ' b wiry r.oots of the pampas-grass. U ndcr .'nch circumstance .. the existence of an u~protect~d tree. i~ impossible. The only pl~nts that hold thmr own, m add1t10n to the indestructible th1s~les, ?r~sscs, and clover, arc a little herbaceous oxalis, produci~ 1g VIviparous buds of extraordinary vitality, a few poisonous spec~es, such ~s the hemlocl~, and a few tough, thorny dwarfacacias and Wiry rushes, whiCh even a starvinrr rat refuse·. "Although the cattle arc a modern i1~troduction the numberless indi.genous r?dents must alwa.ys have effectually prevented the m~roductwn of any other species of plants ; lar~e tra?ts are ~till honeycombed by the ubiquitous bi cacho, a g1gantic rabbit; and numerous other rodents still exist includ~ ng rats and mice, pampas-hares, and the great nutria 'aud earpmcho (capybara) on the river banks."l . Mr. Clark. further rcm~~rks on the de ·perate struggle for exhis tenc.e whiCh charactenses the bor<lerincr fertile zone~ b ., w ~re rivers and marshy plains permit a more luxuriant and vaned vegetable and animal life. After describing how the 1 A Visit to South Anwrica, 1878 ; also Natzwe, vo1. xxxi. pp. 263-339. |