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Show 228 ISLAND LIFE. (PAin' I. . l 'd between these and it is, at all events, IS near y mi way ' . satisfactory that the various meas~res result m figures of the same order of magnitude, which IS all one can expect on so difficult and exceedingly speculative a subject. . The only value of such estimates is to define our n?t10ns of geo l og1· 0a 1 tI' me , and to show that . the enormous p. enods, of hundreds of millions of years, wh10h have sometimes been indicated by geologists, are neither necessary nor warranted by the facts at our command; w hi~e the prese~~ result plac~s us more in harmony with the calculatiOns of physicists, by leavmg a wide marain between aeological time as defined by the very b b • • d h' l fossiliferous rocks, and that far more extensive peno w IC l includes all possibility of life upon the earth. Concluding Rema.rks.-In the present chapter I have end~avoured to show that, combining the measured rate of denudat10n with the estimated thickness and probable extent of the known series of sedimentary rocks, we may arrive at a rude estimate of the time occupied in the formation of those rocks. From another point of departure-that of the proba~le date of. t.he Miocene period, as determined by the. epoch of hig~ excent.nc:ty supposed to have aided in the product10~ of the A1pme ~la01at10n during that period, and taking the estimate of geologists a~ to the proportionate amount of change in the animal. world smce that epoch-we obtain another estimate of the durat10n of geological time, which, though founded on far less secure data, ag_rees pretty nearly with the former estimate. . The tim~ thu~ arnved at is immensely less than the usual estimates of geologists, and is so far within the limits of the duration of the earth as calculated by Sir William Thomson, as to allow for the development of the lower organisms an amount of time anterior to the Cambrian period several times greater than has elapsed betwe~n that period and the present day. I have further shown that, m the continued mutations of climate produced by high excentricity and opposite phases of precession, even though these did not lead to glacial epochs, we have a motive power well calculated to produce far more rapid organic changes than have hitherto been thought possible; while in the enormous amount of specific variation (as demonstrated in an earlier chapter), we CHAP. X.) MEASUHEMENT OF GEOLOGICAL TIME. 229 have ample material for that power to act upon, so as to keep the organic world in a state of rapid change and development proportioned to the comparatively rapid changes in the earth's surface. We have now finished the series of preliminary studies of the biological conditions and physical· changes which have affected the modification and dispersal of organisms, and have thus brought about their actual distribution on the surface of the earth. These studies will, it is believed, place us in a condition to solve most of the problems presented by the distribution of animals and plants, whenever the necessary facts, both as to their distribution and their affinities, are sufficiently well known; and we now proceed to apply the principles we have established to the interpretation of the phenomena presented by some of the more important and best known of the islands of our globe, limiting ourselves to these for reasons which have been already sufficiently explained in our preface. |
