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Show no ISLAND LIFE. (PAlt'l' I, greater wariness can escape their new foes. We rna! thus ?asily understand how a series of changes may occur at d1staut mtervals each leadino- to the selection and preservation of a special set o' f variations, 0 and thus what was a single speci• es may b ecome transformed into a group of allied species differing from each other in a variety of ways, just as we find them in nature. Among these species, however, there will be some which will have become adapted to very local or special conditions, and will therefore be comparatively few in number and confined to a limited area; while others, retaining the more general characters of the parent form, but with some important change of structure, will be better adapted to succeed in the struggle for existence with other animals, will spread over a wider area, and increase so as to become common species. Sometimes these will acquire such a perfection of organisation by successive favourable modifications that they will be able to spread greatly beyond the range of the parent form. They then become what are termed dominant species, maintaining themselves in vigour and abundance over very wide areas, displacing other species with which they come into competition, and, under still further changes of conditions, becoming the parents of a new sot of diverging species. Definition and Origin of Gene1·a.-As some of the most important and interesting phenomena of distribution relate to genera rather than to single species, it will be well here to explain what is meant by a genus, and how genera are supposed. to arise. A genus is a group of allied species which differs from all other groups in some well marked characters, usually of a structural rather than a superficial nature. Species of one genus usually differ from each other in size, in colour or marking, in the proportions of the limbs or other organs, and in the form and size of such superficial appendages as horns, crests, manes, &c. ; but they generally agree in the form and structure of important organs, as the teeth, the bill, the feet, and the wings. When two groups of species differ from each other constantly in one or more of these latter pa.rticulars they are said to belong to different genera. We have already seen CIIAP, IV.) E i VOLUTION TilE KEY TO DISTRIBUTION. Gl that species . ln vary m these more important as \vell as in tho ore su pcrfi · 1 occ . · 1 Cia characters. If, then, in any part of tho area up1ec by a . to it ll speCies some change of habits becomes useful , a such str t l . . . . b uc ura vanatwns as facilitate the chanO'o will e accumulat d b o Co fi d . e Y natural selection, and when they have be-me xe m th , . . h 11 h e proportiOns most boneficml to the animal we s a ave th e fi rs t spec1.0 s of a now genus. ' A creature which has b th d'fi . . h . een us mo 1 ed m Important c 1a ract.e rs will be a new i ype, speci. ally adapted to fill its f ace 1~ the economy of nature. It will almost certainly lave ansen from .an ex~ensive or dominant group, because only such are suffiCiently ncb in individuals to afford an ample s~pply of the necessary variations, and it will inherit the vigo~r. of co~stitution and adaptability to a wide range of cond1t10ns whiCh gave success to its ancestors. It will there~ ore have every c~ance in its ~avour in the struggle for existence; It m~y spread Widely and displace many of its nen,rest allies and m doing so will itself become modified superficially and b~come the parent of a number of subordinate species. It Will. now have become a dominant genus, occupying an entire ~ontme~t, o~ per~ap~ even tw~ or more continents, spreading in every d1rect10n tlll It comes m contact with competing forms better ad.apted to t~e diff~rent environments. Such a genus r:1ay co~tmue to ex1st durmg long geological epochs; but the tlme ":Ill generally come when either physical changeR, or competmg forms: or new enemies are too much for it and it begins to lose its supremacy. First one then anothe; of its component species will dwindle away and become extinct till at last only a few species remain. Sometimes these soon f~llow the others and the whole genus dies out, as thousands of O'enera have died out during the long course of the earth's 1ife-h~tory. but . it will al~o ~ometimes happen that a few species wili contmue ~o mamtam themselves in areas where they are removed from the mfluences that exterminated their fellows. ?a~tse of the Extinction of Species.-There is good reason to ?eheve that the most effective agent in the extinction of species IS the pre~sure of other species, whether as enemies or merely as competitors. If therefore any portion of the earth is cut off |
