350 BET. J. T. GTJLICK ON 



tolerably uniform species so long as there continues a tolerably 

 uniform set of conditions in which it may exist." (See Spencer's 

 ' Principles of Biology,' §§ 91, 156, 169, 170.) In other words, 

 divergence of character in the descendants of one stock occupying 

 different districts does not arise except as it is preceded by dif- 

 ference in the physical conditions, or in the surrounding organisms, 

 of "the different districts. After moulding this thought in many 

 forms, Spencer makes it the fundamental principle on which he 

 builds not a small portion of his philosophy. Darwin is more 

 guarded in his statements ; stiJl, as we have already shown, he 

 sometimes seems to reason from an assumption quite in accord 

 with what Spencer would have us receive as essential to the 

 very idea of causation in vital processes. For example, his expla- 

 nation of the fact that on the different islands of the Gralapagos 

 Archipelago one genus is, in many cases, represented by several 

 closely allied species which are undoubtedly modified forms of 

 one continental species, seems to rest on the assumption that 

 if every species that gained access to any island had at the 

 same time gained access to the other islands of the archi- 

 pelago, there would then have been no occasion or opportunity 

 for the divergences we now find (see ' Origin of Species,' 6th ed. 

 p. 355). 



It seems to me that the divergences presented by the varieties 

 and species of the subfamily AchafinellincB of the Sandwich Islands 

 are at variance with this assumption. Not only are islands in 

 sight of each other occupied by divergent species, but different 

 parts of the same mountain-range, exposed to the same winds 

 and rains and clothed by the same vegetation, are the homes of 

 divergent forms. 



Turning to the map of the island of Oahu, we find a mountain- 

 range extending 36 miles from north-west to south-east nearly 

 parallel with the north-east coast. The north-east side of this 

 range is exposed to the trade-winds fresh from the ocean, and 

 accordingly receives a heavier rainfall than the other side ; but 

 there is not much difference in the amount of rain received by 

 the different valleys on one side of the mountain. In nearly all 

 these valleys on either side of the range are found shady groves 

 of what the natives call the " kukui " {Aleurites triloba). Many 

 species of the subgenera Achatinella and Bulimella have their 

 haunts in these groves, some species clinging to the leaves and 

 young branches, and others to the old branches and trunks. Most 



