Inconsistencies of Utilitarianism . 133 
the preservation of the most fertile individuals , sterility is 
always liable to arise So long as a species remains 
undivided and in occupation of a continuous area its fertility 
is kept up by natural selection ; but the moment it becomes 
separated, either by geographical or selective isolation, or by 
diversity of station or of habits, while each portion must be 
kept jertile inter se, there is nothing to prevent infertility 
arising between the two separated portions . As the two por- 
tions will necessarily exist under somewhat different conditions 
of life, and will usually have acquired some diversity of form 
and colour — both which circumstances we know to be either the 
cause of infertility or to be correlated with it — the fact of some 
degree of infertility usually appearing between closely allied 
but locally or physiologically segregated species is exactly 
whatw T e should expect ” (pp. 184-185) . Notwithstanding this 
statement he does not seem to have grasped the idea that in 
the geographically isolated portions as well as in the others 
the u different conditions of life ” of which he speaks may be 
the different relations to the environment into which the 
separated portions are brought by their divergent habits, with- 
out any reference to inevitable differences in the size and con- 
tours of the different areas, or in any other features of the 
environments, and that the divergence in the habits may be 
directly due to the prevention of interbreeding between sepa- 
rated portions which inevitably differ in average character, 
especially if they are very small portions. 
Isolated portions differ in varying degrees from the 
average character of the Species . 
The italicised portion of the passage last quoted attributes 
to isolation, in stronger language than I should be willing to 
use, a direct influence in producing divergence in the adjust- 
ments on wdiich fertility in the different portions of the species 
depends. I should prefer to say that in some species the 
adjustments on which fertility depends are so delicate that 
adjustments producing perfect fertility within one intergene- 
rating portion of the species will not produce fertility in 
another portion that has been long isolated. I do not make 
my statements so sweeping as his concerning the divergent 
influence of isolation on any one class of characters, but I 
include all classes of inheritable characters, in sexually pro- 
ducing organisms, as coming under its influence. I also insist 
that the direct influence of isolation in producing divergence 
is in proportion to the degree of segregation, which varies 
immensely in different forms of isolation which are equally 
