OCR Text |
Show 40G GEOGRAPHICAL DISTRIBUTION. CHAP. XII. quarters of the world, where they ~ay have b~c_ome slightly modified in relation to t~eu new conditi~ns. There is also some reason to beheve from geological evidenc: that' organisms low in the scale within each great class, generally change at a slower rate t~an the higher forms; and consequently the lower forms will have had a better chance of ranging widely and of still retaining the same specific character. This fact, together with the seeds and eggs of many low forms being very minute and better fitted for distant transportation, probably accounts for a law which has long been observed, and which has lately been admirably discussed by Alph. de Candolle in regard to plants, namely, that the lower any group of organisms is, the more widely it is apt to range. The relations just discussed,-namely, low and slowlychanging organisms ranging more widely than the hicrh -some of the species of widely-ranging genera 0 ' . themselves ranging widely,-such facts, as alpine, lacus-trine, and marsh productions being related (with the exceptions before specified) to those on the surrounding low lands and dry lands, though these stations are so different-the very close relation of the distinct species which inhabit the islets of the same archipelago,-and especially the striking relation of the inhabitants of each whole archipelago or island to those of the nearest mainland,-are, I think, utterly inexplicable on the ordinary view of the independent creation of ea~h species, but are explicable on the view of colon:satwn from the nearest and readiest source, together with the subsequent modification and better adaptation of the colonists to their new homes. Summary of last and present Ohapters.-In these chapters I have endeavoured to show, that if we make due allowance for our ignorance of the full effects of all CIIAP. XII. SUl\iMAHY. 107 the dhanges of climate and of the level of the 1 d wh I.C h h ave certai.n ly occurred within the recent pe a·n d' d f h . . no , an o ot er similar changes which may have occur d W.l t hI' n th. e · same period ; if we remember how prreo - foundly. Ignorant we are with respect to the many an~ cunous means of occasional transport,-a subject whic~ has hardl! ev~r been properly experimentised on; If we b~ar In mind how often a species may have ranged continuously over a wide area, and then have become extinct in the in termeclia te tracts I think the difficulties in believing th~t all the individuals of the same species, wherever located, have desce~ded from the same parents, are not insuperable. And we are led to this co~clusion, which h~s be~n arrived at by many na.tu~ahsts under the designation of single centres of creatwn, ~y some general considerations, more especially fr_om. the. Importance of barriers and from the analogical distnbutwn of sub-genera, genera, and families. ~ith respect to the distinct species of the same genus, whwh on. my theory must have spread from one parentsour~ e ; if we make the same allowances as before for our Ignorance, and remember that some forms of life change most slowly, enormous periods of time being t~us gr~nted for their migration, I do not think that the drf~iculties are. insuperable; though they often are in t~Is case, and 1n that of the individuals of the same speCies, extremely grave. . A~ ex~mplifying the effects of climatal changes on distribution, I .have attempted to show how important ha~ been the Influence of the modern Glacial period, which I am fully convinced simultaneously affected the whol? world, or at least great meridional belts. As showing how diversified are the means of occasional transport, ~ have discussed at some little length the means of dispersal of fresh-water productions. · |