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ould have inhabited the surrounding and differently circumstanced districts； will be prevented。 But isolation probably acts more efficiently in checking the immigration of better adapted organisms； after any physical change； such as of climate or elevation of the land； &c。； and thus new places in the natural economy of the country are left open for the old inhabitants to struggle for； and become adapted to； through modifications in their structure and constitution。 Lastly； isolation； by checking immigration and consequently competition； will give time for any new variety to be slowly improved； and this may sometimes be of importance in the production of new species。 If； however； an isolated area be very small； either from being surrounded by barriers； or from having very peculiar physical conditions； the total number of the individuals supported on it will necessarily be very small； and fewness of individuals will greatly retard the production of new species through natural selection； by decreasing the chance of the appearance of favourable variations。
If we turn to nature to test the truth of these remarks； and look at any small isolated area； such as an oceanic island； although the total number of the species inhabiting it； will be found to be small； as we shall see in our chapter on geographical distribution； yet of these species a very large proportion are endemic； that is； have been produced there； and nowhere else。 Hence an oceanic island at first sight seems to have been highly favourable for the production of new species。 But we may thus greatly deceive ourselves； for to ascertain whether a small isolated area； or a large open area like a continent； has been most favourable for the production of new organic forms； we ought to make the comparison within equal times； and this we are incapable of doing。
Although I do not doubt that isolation is of considerable importance in the production of new species； on the whole I am inclined to believe that largeness of area is of more importance； more especially in the production of species； which will prove capable of enduring for a long period； and of spreading widely。 Throughout a great and open area； not only will there be a better chance of favourable variations arising from the large number of individuals of the same species there supported； but the conditions of life are infinitely complex from the large number of already existing species； and if some of these many species become modified and improved； others will have to be improved in a corresponding degree or they will be exterminated。 Each new form； also； as soon as it has been much improved； will be able to spread over the open and continuous area； and will thus come into competition with many others。 Hence more new places will be formed； and the competition to fill them will be more severe； on a large than on a small and isolated area。 Moreover； great areas； though now continuous； owing to oscillations of level； will often have recently existed in a broken condition； so that the good effects of isolation will generally； to a certain extent； have concurred。 Finally； I conclude that； although small isolated areas probably have been in some respects highly favourable for the production of new species； yet that the course of modification will generally have been more rapid on large areas； and what is more important； that the new forms produced on large areas； which already have been victorious over many competitors； will be those that will spread most widely； will give rise to most new varieties and species； and will thus play an important part in the changing history of the organic world。
We can； perhaps； on these views； understand some facts which will be again alluded to in our chapter on geographical distribution； for instance； that the productions of the smaller continent of Australia have formerly yielded； and apparently are now yielding； before those of the larger Europaeo…Asiatic area。 Thus； also； it is that continental productions have everywhere become so largely naturalised on islands。 On a small island； the race for life will have been less severe； and there will have been less modification and less extermination。 Hence； perhaps； it comes that the flora of Madeira； according to Oswald Heer； resembles the extinct tertiary flora of Europe。 All fresh…water basins； taken together； make a small area compared with that of the sea or of the land； and； consequently； the competition between fresh…water productions will have been less severe than elsewhere； new forms will have been more slowly formed； 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 we find some of the most anomalous forms now known in the world； as the Ornithorhynchus and Lepidosiren； which； like fossils； connect to a certain extent orders now widely separated in the natural scale。 These anomalous forms may almost be called living fossils； they have endured to the present day； from having inhabited a confined area； and from having thus been exposed to less severe competition。
To sum up the circumstances favourable and unfavourable to natural selection； as far as the extreme intricacy of the subject permits。 I conclude； looking to the future； that for terrestrial productions a large continental area； which will probably undergo many oscillations of level； and which consequently will exist for long periods in a broken condition； will be the most favourable for the production of many new forms of life； likely to endure long and to spread widely。 For the area will first have existed as a continent； and the inhabitants； at this period numerous in individuals and kinds； will have been subjected to very severe competition。 When converted by subsidence into large separate islands； there will still exist many individuals of the same species on each island： intercrossing on the confines of the range of each species will thus be checked： after physical changes of any kind； immigration will be prevented； so that new places in the 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 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