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Show 104 DARWINJSI\L CIIAP. throughout all geological time and in every part of the world. Land and water have been contimnlly shifLing their po. itions; some rcu·ions arc undergoing subsidence with diminution of area, others elevation with extension of area; dry hnd has been converted into marshes, while marshes hiwc been drain ed or have even been elevated into phtcaux. Clima.tc too ha~ changed again and a.gitin, either through the cl vation of mountains in high btitudcs leading to the accnmulation of snow and icc, or by a change in the direction of winds and ocean currents produced by the subsidence or elevation of lands which connected continents and divided ocean.. Again, al ong with all these changes have come 11ot loss important changr.in the distribution of species. v co·cta.tion hn.s been gr 'at ly modified by changes of climate and of altitude; while c\·cry union of lands before separated ha. led to extensive mio·mt iom; of animals into new countries, disturbing the bn.hncc t hitt before existed among its forms of life, leading to the oxtcrnJina.tion of some species and tho increase of others. \Vhcn such physical changes as these lmvc tn.kcn place, it is evident that many species must either become moclili c<l or cease to exist. When the vegetation has changed in chamtLcr the herbivorous animals must become able to live on new a.nd perhaps less nutritious food; while the change from a chlup to a dry climate may necessitate migration at certain peri od:-; to escape destruction by drought. This will expose the species to new dangers, and require special modifications of strnctnrc to meet them. Greater swiftness, increased cunning, nocLtlrnal habits, change of colour, or the power of climbing trees a.n<l living for a time on their foliage or fruit, may be the means adopted by different species to bring thcmsel vcs into harmony with the new conditions ; and by the continued survival of those individuals, only, which varied suff-iciently in the ri uht direction, the necessary modifications of . tructurc or of ft;~I ction would be brought about, just as surely as man has lJecn able to breed the greyhound to hunt by sight and the fox hound by scent, or has produced from the same wild plant such distinct forms as the cauliflower and tho bm sols sproul R. Wo will now consider the special charactcri tics of the changes in species that are likely to be effected, and how ftl i' they agree with what we observe in nature. v NATURAL HU:LI~ 'TION 105 Dive1·gmre of ( ?~amcter. In . pecic which have a wide range the struggl~ f~r _cxist' ttcc will often cause ·ome inclividwd. or groupf; of mdJviclnal.' to a.dopt new habits in order to :oize upon va.ca~t. phcc. in nature whore the stmg<rle i.- less severe. Home, hvmg am?ng ext 'n i vo mar:-;hcs, may adopt n. more a.qnatic mode of bfc ; other.-, living whore fore. ts ithound, may lJocomc more arhorc1d. ln either case we cannot doubt that tho cha.ngcs of structure noctlcd to adapt them to th eir new hn.hit.s :vonlc~ soon he brou1rht <Lhout, bccau.-c we know thil.t va.nat10ns lll n.ll the extc1~ml orga.n. and all their cpa.mtc parts arc vm_'Y ahnncla.nt and arc :.dso considerable in amount. Thn,t such ch vcrg ·ucc of character has actually occnrrcd we have some direct. vi~l nco. Mr. Da.rwin informs us that in th Catskill Monntam. 1~1 the U nitc<l 'tatcs there arc two varicti e. of wolves, one w1th a licrht crrcyhoun<l-1ikc form which punmc door, the oth •r more b~lkyb with shorter ]co·:;, which more fi_'C<]U ntl_y attac~s shecp.l Another goocl example i that of the 111 ·cots ~n t~c Island of Madeira, mitny of which have either lost th01r :vmgs o~ ha.vc had them so much reduced as to he uscles. for fhght, wlulc the very same species on the continent of. Europe po : c s fu~ly developed wings. In other ca. cs the '~mcrlcss M~tdc1m: pcc1cs arc distinct from, hnt clo. ely allied to, wmgcd sp~c10s ~£ },UI·opc. The cxpb.nation of this cJuuwc i., that _Ma.dmra, hkc many oceanic islands in the temperate zone, 1s n:uch OXI?oscd to sudden gales of wind, and as mo:;t of tho fortrlc lm.rd 1s on the coast, insects which flew much wmtlcl he very hahlc to he blown out to sea and lost. Y car after year, therefore, those individuals which had shorter wings, or which used th em le~ t, were preserved; ~tnd thu , i11 ~imc, tone. trial, win <rl ess, or,n~pcrfcctly win(fcd races or spcc10s. ha.:e been pro~luccd. 'I hat this is the tmc explanation of Lh1 • smgnlm· fact 1' proved by much corroborative evidence. There arc some few flow_crfrcquenting insects in Madeira to whom wings ar:e e ·cntwl, and in the c the winas arc somewhat larger than m the S<tmc species on the m~tinlabnd. We _thns ~co that ~here i.-no~ n_cr~~ tendency to the abortion of wmgs m Madcrra, bnt th,_tt 1t JS simply a case of adaptation to new conditions. Those 111s et c; 1 Uri:;in 11' SojJI'Cics, p. 71. |