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Show 34 DARWINISM CHAP. meant the missel-thrush has increased in numbers in Scotla~d durin<; the last thirty years, and this has caused ~ decrease lll the n~1mbers of the closely allied song-thrush m the s:un t The black rat (Mus rattus) was the common rat of conn ry. . h th t tl E nropc t 1'l l , m· the beba1 'nninba of the 01ba teen con urLy , 10 l m.g c br.o wn I.a t (Mus dccumanus) appe.a red .1o1 n· tho_. .. ow.e lrl V ol g<t, an d the n Ce- Sprc.·~, d more or less ra1)ldly t1 1t .o vc.u ,m ,t E.: .ourope, an d bae n01·ally . drove out the b. lack r·a t, wb1Tcbh · m· m.o s1t parts is now comparatively rare or qmte extmct. 1s mv,tt - m· g rat h as n ow been carried by commerc.e all over the. world, and in Now Zealand bas completely cxt1:pated a native r:t~, which the Maoris allege they brought w1th them fr?m th e~r home in the Pacific; and in the same country a natiVe. fly 1:-> beina supplanted by the European house-fly. In _Russia .the smali Asiatic cockroach has driven away a ~arger nat:ve ~peciC:; and in Australia the imported hive-bee 1s extermmatmg the small stincrless native bee. The re~. on why this kind of struagle goes on is apparen t if we consider that the allied species fill nearly tho sa.mc place in the economy of nature. They require .nearly the same kind of food, arc exposed to the same enemies and the :;uno danacrs. Hence if one has ever so slight an advn,ntagc OYer the bother in pr~curing food or in. avoi<li~1g ?an?cr? in its rapidity of multiplication or its tenaCJty. of hfc, 1t w1ll mer asc more rapidly, and by that very fact w1ll. cause the other to J.ccrease and often become altogether extmct. In some c:1scs, no doubt, there is actual war between the two, the strung<'l' killina the weaker; but this is by no means necessary, and there bmay be cases in which the weakc_r spcci~s, ph:y_sicnJl_.\, may prevail, by its power of more ra~nd mult1pl~catiOn , 1l:-> better withstanding vicissitudes of chmatc., or 1ts grca.ter cunning in escaping the attacks of the common c nem1~s. The sa,mc principle is seen at work in the fact tha,t ccrt:t:n mountn,in varieties of sheep will starve out othC1' mounta,m varieties, so that the two cannot be kept together. In plant!:i the same tbina occurs. If several distinct vn,rictics of wheat b • 1 are sown together, and the mixed seed resown, some of t 10 varieties which best suit the soil and climute, or arc rmturally the most fertile, will beat the others and so yield more scccl, aml will consequently in a few years supplant the other varieties. II THE STRUGGLE FOR EXISTENCE 35 ~s an effcc.t of this principle, we seldom find closely allied SI?C~ICs of ammals or plants living together, but often in d1stmct though adjacent districts where the conditions of lifo are. somcw ~at ~iffcrcnt. Thus we may find cowslips (Primula vc~t~) ~rowmg m a m.eadow, and primroses (P. vulgaris) in an adJOllllng wood, each m abundance, but not often intermingled. And. for the same reason the old turf of a pasture or heath consists of a great variety of plants matted torrcthcr so much so that in a patch little more than a yard squ~rc M~·. Darwin found twenty ~istinct species, belonging to eighteen distinct 1 genera and to CLght natural orders, thus showina their extreme • diversity of organisation. For the same reas~n a number of distinc.t grasses and clovers are .sown in order to make a good lawn mstead of any one spec10s; and the quantity of hay p~o~uced has been found to be greater from a variety of very d1stmct grasses than from any one species of grass. It ~ay ?c thought that forests arc an exception to this rule, s~nce m the no~·th-tempcrate and arctic regions we find cxtens1ve forests of pmes or of oaks. But these are after all e~ceptio~al, .and characterise those regions only ,~here th~ chm~te 1s httlc favourable to forest vegetation. In the trop1cal and all tho warm temperate parts of the earth where there is a. sufficient s~pply of moisture, the forests prc~cnt the same. vancty of sp~Cles. a~ docs the turf of our old pastures; and m the cquatonal v1rgm forests th rc is so great a variety of forms, and they arc so thoroughly intermingled, that the traveller oft~n finds it .difficul~ to discover a second specimen of any particular spcCJes whiCh he has noticed. Even the forests of the temperate zones, in all favourable situation exhibit a considerable variety of trees of distinct aenera ancl families, and it is only when we approach the o~tskirts of forest vc~etati~n, where either drought or winds or the severity of the Winter 1s ad verse to the existence of most trees, that we find extensive tracts monopolised by one or two species. Even Canada bas more than sixty different forest trees and the Eastern U nite.d. States a hundred and fifty ; Euroi)c is rather poor, contam~ng about eighty trees only; while the f~rests of Eastern Asm, Japan, and Manchuria are exceedingly ncb, about a hundred and seventy species being already known. And in all these countries the trees grow inter- |
