82 THE ORIGIN OF SPECIES 



generally be almost uniform; so that natural selection will tend 

 to modify all the varying individuals of the same species in the 

 same manner. Intercrossing with the inhabitants of the surround- 

 ing districts will also be thus prevented. Moritz Wagner has lately 

 published an interesting essay on this subject, and has shown 

 that the service rendered by isolation in preventing crosses be- 

 tween newly-formed varieties is probably greater even than I sup- 

 posed. But from reasons already assigned I can by no means agree 

 with this naturalist, that migration and isolation are necessary ele- 

 ments for the formation of new species. The importance of isolation 

 is likewise great in preventing, after any physical change in the 

 conditions, such as of climate, elevation of the land, etc., the immi- 

 gration 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 

 sometimes 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 favorable variations arising. 



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

 against natural selection. I state this because it has been errone- 

 ously 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 impor- 

 tance in this respect is great, that it gives a better chance of bene- 

 ficial 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 chap- 

 ter 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 favorable 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 con- 

 tinent, has been most favorable for the prodi ~tion of new organic 



