OCR Text |
Show 100 CIRCUMSTANCES FAVOURABLE [CHAP. IV Heer, resembles the extinct flora of Europe. All freshwater basins taken together, make a small area compared with that of the sea or of the land; and, ?onseq_uently, the competition between fresh-water productions ~1ll have been less severe than elsewhere; new forms w1ll have been more slowly forrned, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order : and in fresh water -y;e find some of the ~ost anomalous forms now known In. the ~orld, a~ the Ornithorhynchus and Lepidosiren, whwh, hke fossils, connect to a certain extent orders now widely separated in the na.t11:ral scale. These anomalous forms may almost be called hving fossils; they have endured to the present d~y, from havinO' inhabited a confined area, and from having thus been 5 - .• exposed to less severe con1petition. To sun1 up the circumstances favourable and ll:nfayourable to natural selection, as far as the e:rtreme Intricacy of the subject permits. I conclude, looking t? the future, that for terrestrial productions a large .con~mental area, which will probably undergo many oscillations .of leyel, and which consequently will exist for lonO' periods In a broken condition will be the most favourab1e for the production of many' new forms of life, lil~ely to endure .long and to spread widely. Fo~ the ~rea ·will first _have ~x1sted as a continent and the Inhabitants, at this per1~d numerous in indi;iduals and kinds, will have been sub.Jecte~ to very severe competition. When convert~d b:y sub~Idence into large separate islands,. there will. st1ll ~x~st many individuals of the same spemes on each Island . ~ntercrossing on the confines of the range of each spe~1es will thus be checked : after physical changes of any .kind, immigration will be prevented, so that new places In t)le polity of each island will have to be .filled .up by modifications of the old inhabitants; and time will be allowed for the varieties in each to become well modified and perfected. When, by renewed elevation, the is:ands .shall be reconverted into a continental area, there will again b.e severe competition: the most favoured or_improved varieties will be enabled to spread : there Will be much ex· Cn.AP. IV.] TO NATURAL SELECTION. 101 tinction of the less improved forms, and the relative proportional numbers of the various inhabitants of the renewed continent will again be changed ; and again there will be a fair field for natural selection to improve still further the inhabitants, and thus produce new species. That natural selection will always act with extreme slowness, I fully admit. Its action depends on there being places in the polity of nature, which can be better occupied by some of the inhabitants of the country undergoing modification of some kind. The existence of such places will often depend on physical changes, which are generally very slow, and on the immigration of better adapted forms having been checked. But the action of natural selection will probably still oftener depend on some of the inhabitants becoming slowly modified ; the mutual relations of many of the other inhabitants being thus disturbed. Nothing can be ~ff'ected, unless favourable variations occur, and variation itself is apparently always a very slow process. The process will often be greatly retarded by free intercrossing. Many will exclaim that these several causes are amply sufficient wholly to stop the action of natural selection. I do not believe so. On the other hand, I do believe that natural selection will always act very slowly, often only at long intervals of time, and generally on only a very few of the inhabitants of the same region at the same tiine. I further believe, that this very slow, intermittent action of natural selection accords perfectly well with what geology tells us of the rate and manner at which the inhabitants of this world have changed. Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change, to the beauty and infinite complexity of the coadaptations between all organic beings, one with another and with their physical conditions of life, which may be effected in the long course of time by nature's power of selection. Extinotion.-This subject will be more fully discussed |