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Show 18 DARWINISM CIIAl'. so as to keep out cattle. On ascertaining this, Mr. D:trw~ n was so much snrpri cd that he searched among .the hoathc.r 111 the unenclosed parts, and thoro he found mult1tudcs of l1tt le trees and seedlino·s which bad been perpetually browsed clown by tho cattle. Inb one square yard, at a point about a hnn ~lre<l yards from one of the old. clumps of firs, he cou~tcd. .thi rty two little trees and one of them had twonty-s1x rmgs of growth, showing that it had for many years tried t~ raise iLs head above the stems of tho heather and had fa.1led. Y ct this heath was very extensive and very barren, and, as Mr. Darwin remarks, no one would ever have imagined that cattle would have so closely and so effectually searched it for food. In the case of animals, the competition and strngglc arc more obvious. The vegetation of a given district can only support a certain numbee of animals, and the diO'crent kinds of plant-eaters will compete together for it. They will :tl :so have insects for their competitors, and these insects will he kept down by birds, which will thus assist the mamma.li il. But there will also be carnivora. destroying the herLivora ; while small rodents, like the lemming and some of the li eld· mice, often destroy so much vegetation as materially to aflcd the food of all the other groups of animals. Droughts, fl ood ~, severe winters, storms and hurricanes will injure these in various ucgrecs, but no one species can be uimini shcd in numbers without the efl'ect being felt in various complex ways by all the rest. A few illustrations of this reciprocal action must be given. Illustrative Cases of the Stntggle for Life. Sir Charles Lyell observes that if, by the attacks of sea.]:.; or other marine foes, salmon are reduced in numbers, Lhc consequence will be that otters, living far inland, will he deprived of food and will then destroy many young bird · 01 quadrupeds, so that the increase of a. marine animal may cause the destruction of many land animals hundrcus of mil es away. Mr. Darwin carefully observed the effects prodncc(1 by planting a few hundred acres of Scotch fir, in Stafforushirc, on part of a very extensive heath which had never been cultivated. After the planted portion was about twenty-fi ve years old he observed that the change in the native vegetation II TilE STRUGGLE FOR EXISTENCE 19 ~a.~ greatm: than is often sec~ in pnssing from one quite d1fie~ent so1l to another. Be:<si<lcs :t gt·c:lt change in the propor. tJOnal numb0rs of the n:ttlvc heath-plants, twelve species wh1ch could not be found on the heath flourished in the plantations. The cff'cct on the in ect life mu. t have been still weater, for si~ insectivorous hirds which were very common m the plantat10ns were not to be seen on the heath, which ~as, ~owever, ~rcqucntcd by two or three different specie of msect1vorons b1rds. It woul~ have required continued study for several years to determmc all the difl'ercnccs in th organ~c life of th.e two areas, but the facts st~ttcd lly Mr. Darwm are su~c10nt to. show how great a change may he effected by the mtroductwn of a inglc kind of tree and th, keeping out of cattle. The next case I will give in Mr. Darwin's own word:: "In several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor hor. es nor doo·s hn.vc ever run wild, though they swarm southw:ml and northward in a feral state; and Azara. and Hcngger have shown that this i ~ ca.u.scd by t.hc grcat~r numbers, in Paragnay, of a certain fly whJCh lays 1ts c(J'gs m the navels of thc:c a11imals when first born. The increase of these flies, nnmcrous as they arc, must .b.e h.abitually checked. by some means, probably hy other parasitic msects. Hence, 1f cert:tin in. eetivorou. birds were ~o decrease in I a~·aguay, the parasitic insect: would probal,iy mcrcase; and th1s would lessen the nnmbcr of the 11avclfrcque~ ting flies-then cattle and horses would become feral, and th1s would greatly alter (as inclcc<l I have observed in parts of South America) the vegetation : this again would 1~rg~ly aff?ct the ~nsect~, and thi., as we have just seen in Staffordshire, tho mscct1vorous Lirds, and so onward in everincreasing circles of complexity. Not that under nature tho relations ·will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long run the forces arc so nicely balanced, that the face of nature rc~ains for a l~ng time .uniform, though assuredly the merest tnfie would g1 vc the VIctory to one organic being over another." I 1 The Origin of ..'.S pecies, p. 56. |