Inco?isistencies of Utilitarianism. 133 



the preservation of the most fertile individuals^ sterility is 



ahvays liahle to arise So long as a species remains 



undivided and in occupation of a continuous area its fertility 

 is hept up ly natural selection; hut the moment it becomes 

 separated, either hy geographical or selective isolation, or by 

 diversity of station or of habits, ichile each portion must he 

 Tiept 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 

 what we 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 " 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 tlie 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 which 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- 

 ratirjg 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 



