498 EEV. J. '£. GULICK ON DIVEftSITY Or EYOLUTION 



east side of the mountain is somewhat heavier than on the 

 opposite side, and the higher ridges of the mountains are cooler 

 than the valleys ; but the valleys on one side of the range have a 

 climate the same in every respect. The vegetation in the valleys 

 differs somewhat from that on the ridges ; but the vegetation of 

 the different valleys is much the same ; the birds, insects, and 

 larger animals are the same. Though, as far as we can observe, 

 the conditions are the same in the valleys on one side of the range, 

 each has a molluscan fauna diff'ering in some degree from that of 

 any other. We frequently find a genus represented in several 

 successive valleys by allied species, sometimes feeding on tlie same, 

 sometimes on different plants. In every sucli case, the valleys 

 that are nearest to each other furuisli the moat nearly allied 

 forms ; and a full set of the varieties of each species presents a 

 minute gradation of forms between the more divergent types 

 found in the more widely separated localities. 



No theory is satisfactory that does not account, 1st, for their 

 being distributed according to their affinities in adjoining areas 

 more or less distinctly defined, and, 2nd, for their being restricted 

 to very small areas. 



External Conditions not the Cause. 



I think the evolution of these diflfereut forms cannot be attri- 

 buted to diff'erence in their external conditions : — 



1st. Because in different valleys, on the same side of the moun- 

 tain, where food, climate, and enemies are the same, there is still 

 a difference in the species. 



2nd. Because we ffnd no greater diflference in the species 

 "when we pass from the more rainy to the drier side," than when 

 we compare the forms from valleys on the same side of the moun- 

 tain, separated by an equal distance. 



3rd. Because if, failing to find a reason in the more manifest 

 conditions, we attribute the difference in the species to occult 

 influences, such as magnetic currents, we must suppose that there 

 are important differences in these hidden conditions for each suc- 

 cessive mile, and that their power at the Sandwich Islands is 

 a thousand times greater than in most countries. 



Separation and Variation Correlative Factors in the JEvolution 



of Species. 

 If we would account for the diff'erence and for the limited dis- 

 tribution of these allied forms on the hypothesis of Evolution 



