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Show 500 ISLAND LIFE. [PART II. kinds-firstly, the constant tendency of all organisms to i~crease in numbers and to occupy a wider area, a.nd the1r various powers of dispersion and migration through whlCh, when h ked they are enabled to spread widely over the globe; unc ec , · · 1 · h and, secondly, those laws of evolution and ex~mcbon. w nc d determine the manner in which groups of orgamsms anse an grow, roach thei. r maximum, a. nd the. n dwindle a.w ay,. often breaking up inLo separate portwns whlCh long surv~ve m very remote reO'ions. The physical causes are also mamly of two kinds. We have, first., the geographical changes which at one time isolate a whole fauna and flora, at another time lead to their dispersal and intermixture with ad~acent faunas and floras -and it was here important to ascertam and define the exact nature and extent of these changes, and to determin~ the question of the general stabilit~ or instability of contme~ts and oceans; in the second place, 1t was necessary to determlD.O the exact nature, extent, and frequency of the changes of ellmate which have occurred in various parts of the earth,because such changes are among the most powerful ager:ts in ca.using the dispersal and extinction of pla~ts and· amm~ls. Hence the importance attached to the questiOn o! geol.oglCal climates and their causes, which have been here mvest1gated at some lenO'th with the aid of the most recent researches of b • • •• geologists,· physicists, and O"'{plorers. These van~us mqmn~s led on to an investigation vf the mode of formatiOn of stratified deposits, with a view to fix within some limits their probable age; and also to an estimate of the probable rate of development of the organic world; and both these processes are shown to involve, in all probability, periods of time less vast than have generally been thought necessary. The numerous facts and theories established in tho First Part of the work are then applied to explain the phenomena presented by the floras and faunas of the c~ief isla.nds of ~he globe, which are classified, in accordance w1th the1r physical origin, in three group or classes, each of which are shown to exhibit certain well-marked biological features. Having thus shown that the work is a 0onnected whole, founded on the principle of tracing out the more rec0ndite CIIAP. XXIV.] SUMMARY AND CONCLUSION. 501 causes of the distribution of organisms, we will briefly indicate th~ scope and object of the several chapters, by means of which th1s general conception has been carried out. Beginning with simple and familiar facts relating to British and European quadrupeds and birds I have defined and shown the .exact character of "areas of distribution," as applied t~ spec1es, genera, and families, and have illustrated the subject by maps showing the peculiarities of distribution of some wellknown groups of birds. Taking then our British mammals and land-birds~ I follow them over the whole area they inhabit, and thus obtam a foundation for the establishment of "zoolo(Tica.l regions," and a clear insight into their character as distinct from the usual geographical divisions of the globe. The facts thus far established are then shown to be necessary results of the "law of evolution." The nature and amount of "vari~t~on" is exhibited by a number of curious examples; the ongm, growth, and decay of species an<.l genera are traced, and all the interesting phenomena of isolated groups and discontinuous generic and specific areas are shown to follow as logical consequences. The next subject investigated is the means by which the various groups of animals are enabled to overcome the natural barriers which often seem to limit them to very restricted areas, how far those barriers are themselves liable to be altered or abolished, and what is the exact nature and amount of the changes 'of sea and land which our earth has underO'one in b past times. This latter part of the inquiry is shown to be the most important as it is the most fundamental; and as it is still a subject of controversy, and many erroneous views prevail in regard to it, it is discussed at some length. Several distinct classes of evidence are adduced to prove that the grand features of our globe-the position of the great oceans and the chief land-areas-have remained, on the whole, unchanged throughout geological time. Our continents are shown to be built up mainly of" shore-deposits;" and even the chalk, which is so often said to be the exact equivalent of the u globigerinaooze" now forming in mid-Atlantic, is shown to be a comparatively shallow-water deposit formed in inland seas, or in |