104 
CIRCUMSTANCES FAVOURABLE 
Chap. IY. 
obviously thus act far more efficiently with those animals 
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 sm-rounding 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- 
