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Show t08 NATURAL SELECTION. CHAP. IV. vented so that new places in the polity of each island will h~ve 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 islands shall be re-converted into a continental area, there will again be severe competition: the most favoured or improved varieties will be enabled to spread: there will be much extinction 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 effected, 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 \vill exclaim that these several causes are a1nply 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 time. I further believe, that this very slow, intermit- CHAP. IV. EX'flNCTI ON. 109 tent action of natural selection accords perfectly well wjth 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 li1nit to the amount of change, to the Leauty and infinite complexity of the coadaptationR 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. Extinction.-This subject will be more fully discussed .in our chapter on Geology; but it must be here alluded to from being intimately connected with natural selection. Natural selection acts solely through the preservation of variations in some way ad,antageous, which consequently endure. But as from the high geometrical powers of increase of all organic beings, each area is already fully stocked with inhabitants, it follows that as each selected and favoured form increases in number, so will the less favoured forms decrease and become tare. Rarity, as geology tells us, is the precursor to extinction. vV e can, also, see that any fonn represented by few individuals will, during fluctuations in the seasons or in the number of its enemies, run a good chance of utter extinction. But we may go further than this; for as new forms are continually and slowly being produced, unless we believe that the number of specific forms goes on perpetually and almost indefinitely increasing, numbers inevitably must become extinct. That the number of specific forms has not indefinitely increased, geology shows us plainly; and indeed we can see reason why they should not have thus increased, for the number of places in the polity of nature is not indefinitely great,-not that we |