104 CIRCUMSTANCES FAVOURABLE Chap. IV. 



which unite for each birth ; but I have already attempted 

 to show that we have reason to believe that occasional 

 intercrosses take place with all animals and with all 

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

 I am convinced that the young thus produced will gain 

 so much in vigour and fertility over the offspring from 

 long-continued self-fertilisation, that they will have a 

 better chance of surviving and propagating their kind ; 

 and thus, in the long run, the influence of intercrosses, 

 even at rare intervals, will be great. If there exist 

 organic beings which never intercross, uniformity of 

 character can be retained amongst them, as long as 

 their conditions of life remain the same, only through 

 the principle of inheritance, and through natural selec- 

 tion destroying any which depart from the proper type ; 

 but if their conditions of life change and they undergo 

 modification, uniformity of character can be given to 

 their modified offspring, solely by natural selection pre- 

 serving the same favourable variations. 



Isolation, also, is an important element in the process 

 of natural selection. In a confined or isolated area, if 

 not very large, the organic and inorganic conditions of 

 life will generally be in a great degree uniform ; so that 

 natural selection will tend to modify all the individuals 

 of a varying species throughout the area in the same 

 manner in relation to the same conditions. Intercrosses, 

 also, with the individuals of the same species, which 

 otherwise would have inhabited the surrounding and dif- 

 ferently circumstanced districts, will be prevented. But 

 isolation probably acts more efficiently in checking the 

 immigration of better adapted organisms, after any phy- 

 sical change, such as of climate or elevation of the land, 

 &c. ; and thus new places in the natural economy of 

 the. country are left open for the old inhabitants to 

 struggle for, and become adapted to, through modifica- 



