128 CIRCUMSTANCES FAVOURABLE TO THE [Chap. IV. 



ing, after any physical change in the conditions, such 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 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 num- 

 ber 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 aris- 

 ing. 



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 impor- 

 tance 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 re- 

 marks, and look at any small isolated area, such as an 

 oceanic island, although the number of species inhabit- 

 ing it is small, as we shall see in our chapter on Geo- 

 graphical 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 



