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Show 506 18LAND LIFE. (PAR'l' IT, . . r these rocesses are either of them important to mqmre whethe p d and I devote a chapter so excessively slow as has been suppose ' to the inq· uiryh. measure d 'th some accuracy the maximum Wl Geolog1stsf 1a1 vet he known se dl'i D entary rocks. The rate. of thicknes.s o h a lso been recen tlY measured by a m.e thod whiCh, denudatwn · as at all events g1. ve s results of the nght order of if not. prde Cise,d aw hich err on th e side of beinO' too slow rather o . magmtu e an h ·m~~~m thickness of the lcnown f t If then t e maxt w than too as · k '·ks ta' e n t o re present the average thickness sedimentary roc s 1 1 d we also know the amount of h d. ntary roc {!:"!, an · · of a. ll t e se I· mde to the sea or 1a kes , and the area over. whwh sed1ment carne d h e a means of calculatmg the d. t · sprea we av that se nnen lS ' •.1d. of all the .sedimentary rocks · d £ the bm lUO' up · time reqmre or t have here inquired how far the of the geological system. t ron which side they probably 't' ons are correc , o · . . n.bove supposl 1 · . . d t is that the time reqmred 1s . nd the concluswn arnve a ' err' a has hitherto been supposed. very much less. than . ff d d by the date of the last glacial Another ~sti~ate IS. ah ot~ e last period of high excentricity, h cOinCident Wlt e . . d t epo'c1 thas Al ine glaC.i at.w n of the Miocene penod 1s a.s sume o wh1 e e P d by the nex t ear1 1·er phase of very lngh excen-have been ca~se . d t the proportionate change of the tricity. Takmg theseffasd a a, eans of arriving at the whole · f llusca a or s a m species o mo d b the fossiliferous rocks ; and these 1 f t' me represente Y . apse o 1 . th order of their magmtudes. two e~timates agreedl~h te the chanaes of climate every 10,500 It IS then argue a . dos of hiah excentricity have d · th numerous peno o yeard unng . e . h tening on both geological anJ d otive power m as . . ll ac. te as a m . . d lowering the snow-hne m a 1 . 1 h- e By ra1sma an . bw ogiC.a c Gtng" . t has causeo d m. creas ed denudation.' whlle the mountam ranges 1 d h mi'gration and disturbance in h have cause roue · same c a~ges d have thus tended to the more. rapld the orgamc world, an och beina a penod of modification of species. The prhes~n~ epa pba:e ~f exce11tional t · · t the eart 1s m -r very low excen .nm y' . . . f this period stability both physical and orgamc; t~nd ~/~h:o:ry slow rate of exceptional stability that our no Ions of change have been derived. CHAP. XXIY.] SU:\'lMARY AND CONCLUSION. 507 The conclusion is, on the whole, that the periods allowed by physicists are not only far in excess of such as are required for geological and organic change, but that they allow ample margin for a lapse of time anterior to the deposit of the earliest fossiliferous rocks several times longer than the time which has elapsed since their deposit to the present day. Having thus laid the foundation for a scientific interpretation of the phenomena of distribution, we proceed to the Second Part of our work-the discussion of a series of typical Insular Faunas and Floras with a view to explain the interesting phenomena they present. Taking first two North Atlantic groups-the Azores and Bermuda, it is shown how important an agent in the dispersal of most animals and plants is a stormy atmosphere; Although 900 and 700 miles respectively from the nearest continents, their productions are very largely identical with those of Europe and America; and, what is more important, fresh arrivals of birds, insects, and plants, are now taking place almost annually. These islands afford, therefore, test examples of the great dispersive powers of certain groups of organisms, and thus serve as a basis on which to found our explanations of many anomalies of distribution. Passing on to the Galapagos we have a group less distant from a continent and of larger area, yet, owing to special conditions, of which the comparatively stormless equatorial atmosphere is the most important, exhibiting far more speciality in its productions than the more distant Azores. • Still, however, its fauna and flora are as unmistakably derived from the American continent as those of the Azores are from the European. We next take St. Helena and the Sandwich Islands, both wonderfully isolated in the midst of vast oceans, and no longer exhibiting in their productions an exclusive affinity to one continent. Here we have to recognise the results of immense antiquity, and of those changes of geography, of climate, and in the general distribution of organisms which we know have occurred in former geological epochs, and whose causes and consequences we have discussed in the first part of our volume. This concludes our review of the Oceanic Islands. Coming now to Continental Islands we consider first those of |