Chap. IV.] RESULTS OP NATURAL SELECTION. 127 



take place only at long intervals of time, the young thus 

 produced will gain so much in vigour and fertility over 

 the ofEspring from long-continued self-fertilisation, that 

 they will have a better chance of surviving and propa- 

 gating their kind; and thus in the long run the influ- 

 ence 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 con- 

 ditions 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 pre- 

 serving similar favourable variations. 



Isolation, also, is an important element in the modifi- 

 cation of species through natural selection. In a con- 

 fined 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 surrounding 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 

 necessary elements for the formation of new species. 

 The importance of isolation is likewise great in prevent- 



