116 ORIGIN OF SPECIES 



and plants. Even if these take place only at long intervals 

 of time, the young thus produced will gain so much in vigour 

 and fertility over the offspring from long-continued self-fer- 

 tilisation, that they vi^ill have a better chance of surviving 

 and propagating their kind ; and thus in the long run the in- 

 fluence of crosses, even at rare intervals, will be great. With 

 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 inheritance, and through natural selection which 

 will destroy any individuals departing from the proper type. 

 If the conditions of life change and the form undergoes modi- 

 fication, uniformity of character can be given to the modified 

 offspring, solely by natural selection preserving similar fa- 

 vourable variations. 



Isolation, also, is an important element in the modification 

 of species through natural selection. In a confined or iso- 

 lated area, if not very large, the organic and inorganic con- 

 ditions of life will generally be almost uniform; so that nat- 

 ural selection will tend to modify all the varying individuals 

 of the same species in the same manner. Intercrossing with 

 the inhabitants of the surrounding districts will, also, be thus 

 prevented. Moritz Wagner has lately published an interest- 

 ing 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 new species. The im- 

 portance of isolation is likewise great in preventing, after 

 any physical change in the conditions such as of climate ele- 

 vation of the land, &c., the immigration of better adapted or- 

 ganisms; 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 



