76 CIRCUMSTANCES FAVOURABLE TO THE [CHAP. IV. 



respect to organic beings extremely low in the scale, which do not 

 propagate sexually, nor conjugate, and which cannot possibly 

 intercross, uniformity of character can be retained by them under 

 the same conditions of life, only through the principle of inherit- 

 ance, and through natural selection which will destroy any 

 individuals departing from the proper type. If the conditions of 

 life change and the form undergoes modification, uniformity of 

 character can be given to the modified offspring, solely by natural 

 selection preserving similar favourable variations. 



Isolation, also, is an important element in the modification of 

 species through 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 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 

 between newly-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 neces- 

 sary elements for the formation of nev/ 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, 

 &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, how- 

 ever, 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 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 

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

 of beneficial variations arising and of their being selected, accumu- 

 lated, 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 



