16 Darwin, and after Darwin. 



still more of having verified its importance as a factor 

 of organic evolution. , 



For, in point of fact, Mr. Gulick was led to his 

 recognition of the principle in question, not by any 

 deductive reasoning from general principles, but by 

 his own particular and detailed observations of the 

 land mollusca of the Sandwich Islands Here there 

 are an immense number of varieties belonging to 

 several genera ; but every variety is restricted, not 

 merely to the same island, but actually to the same 

 valley. Moreover, on tracing this fauna from valley 

 to valley, it is apparent that a slight variation in the 

 occupants of valley 2 as compared with those of the 

 adjacent valley i, becomes more pronounced in the 

 next valley 3, still more so in 4, &c., &c. Thus it 

 was possible, as Mr. Gulick says, roughly to estimate 

 the amount of divergence between the occupants of 

 any two given valleys by measuring the number of 

 miles between them. 



As already stated, I have myself examined his 

 wonder f ul collection of shells, together with a topo- 

 graphical map of the district ; and therefore I am in 

 a position to testify to the great value of Mr. Gulick's 

 work in this connexion, as in that of the utility 

 question previously considered. The variations, which 

 affect scores of species, and themselves eventually run 

 into fully specific distinctions, are all more or less 

 finely graduated as they pass from one isolated 

 region to the next ; and they have reference to 

 changes of form and colour, which in no one case 

 presents any appearance of utility. Therefore and 

 especially in view of the fact that, as far as he could 

 ascertain, the environment in the different valleys was 



