Cbap. IV. KESULTS OF NATURAL SELECTION. IQH 



Isolation, also, is an important clement in the changes 

 effected tlirout^h natural selection. In a confined or isolated 

 area, if not very large, the organic and inorganic conditions of 

 life will generally be almost uniform ; so that natural selection 

 will tend to modify all the varying individuals of the same spe- 

 cies in the same manner. Intercrossing with the inhal)itants 

 of the surrounding districts ^vill, also, be prevented. Moritz 

 Wagner has lately published an interesting essay on this sub- 

 ject, and has shown that the service rendered by isolation in 

 preventing crosses between newly-formed varieties is probably 

 greater even than I have supposed. But from reasons already 

 assigned I can by no means agree with this naturalist, that 

 migration and isolation are necessary for the formation of new 

 species. The importance of isolation is likewise great in pre- 

 venting, after any physical change in the conditions, such as of 

 climate, elevation of the land, etc., the immigration of better- 

 adapted organisms ; and thus new places in the natural econ- 

 cmy of the district are left open for the old inhabitants to strug- 

 gle for and become ada])ted to. Lastly, isolation will give time 

 for a new variety to be slowly improved ; and this may some- 

 times be of importance in the productioTi of new species. If, 

 however, an isolated area be very small, either from being sur- 

 rounded by jjarriers, or from having very peculiar physical con- 

 ditions, the total number of the inlialiitants will be small ; and 

 this will retard the production of new species through natural 

 selection, by decreasing the chances of the appearance of favor- 

 able individual diflerences. 



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-imjiortant part in modifying species, as if 

 all ^vere necessarily undergoing change through the action of 

 some iimate law. Lapse of time is only so fur injportant, and 

 its importance in this respect is great, that it gives a better 

 chaiKMj of beneficial variations arising, being selected, increased, 

 and fixed, in relation to the; slowly-clianging organic and in- 

 organic (•f)iiditions of life. It likewise favors the definite action 

 of the conditions of life. 



If \\c turn to Nature to test the truth of these remarks, and 

 look at any small, isolated area, such as an oceanic island, al- 

 though the number of the dilTt^rent species inhabiting it is small, 

 as we shall see in our chapter on Geographical Distribution; 

 yet of the species a very large proportion are endemic — that is, 



