82 CIRCUMSTANCES FAVOURABLE TO THE [Chap. If 



formed varieties is probably greater even than I supposed. But 

 from reasons already assigned I can by no means agree with this 

 naturalist, that migration and isolation are necessary elements for 

 the formation of new species. The importance of isolation is like- 

 wise great in preventing, after any physical change in the conditions, 

 ouch as of climate, elevation of the land, &c, the immigration of 

 better adapted organisms; and thus new places in the natural 

 economy of the district will be left open to be filled up by the 

 modification of the old inhabitants. Lastly, isolation will give time 

 for a new variety to be improved at a slow rate ; and this may some- 

 times be of much importance. 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 inhabitants 

 will be small ; and this will retard the production of new species 

 through natural selection, by decreasing the chances of favourable 

 variations arising. 



The mere lapse of time by itself does nothing, either for or against 

 natural selection. I state this because it has been erroneously 

 asserted that the element of time has been assumed by me to play 

 an all-important part in modifying species, as if all the forms of life 

 were necessarily undergoing change through some innate law. Lapse 

 of time is only so far important, and its importance in this respect 

 is great, that it gives a better chance of beneficial variations arising, 

 and of their being selected, accumulated, and fixed. It likewise 

 tends to increase the direct action of the physical conditions of life, 

 in relation to the constitution of each organism. 



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 

 number of species inhabiting it is 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 in the world. Hence an oceanic island at first 

 sight seems to have been highly favourable for the production of 

 new species. But we may thus 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 isolation is of great importance in the production of new 

 species, on the whole I am inclined to believe that largeness of area 

 is still more important, especially for the production of species which 

 shall prove capable of enduring for a long period, and cf spreading 

 widely. Throughout a great and open area, not only will there be a 



