UNDER ONE SET OF EXTERNAL CONDITIONS. 



497 



is variation. The words in which the law is expressed imply that 

 there are variations which may be accumulated in different pro- 

 portions according to the differing demands of external conditions. 



What, then, is the effect of these variations when the external 

 conditions remain the same ? Or can it be shown that there is no 

 change in organisms that is not the result of change in external 

 conditions ? Again, if the initiation of change in the organism is 

 through change in the " Environment," by what law is the cessa- 

 tion of change determined ? If change continues in the organism 

 long after the essential conditions of the " Environment " have 

 become stationary, how do we know that it is not perpetual? 

 Does the change, whether transitory or continuous, expend itself 

 in producing from each species placed in the new " Environ- 

 ment " just one new species completely fitted to the conditions ? 

 or may it produce from one stock many that are equally fitted? 

 If the latter, what is the law or condition that determines their 

 number, their affinities, and the size and position of their re- 

 spective areas, as related to each other and to the whole available 

 area ? 



Facts throwing Light on the Subject. 



I believe that in the relations of species to each other as dis- 

 tributed in nature, we shall find light on the subject. I call 

 attention at this time to the variation and distribution of ter- 

 restrial mollusks, more especially those found on the Sandwich 

 Islands ; but similar facts are not wanting elsewhere. 



The land-shells of the Sandwich Islands not only differ in 

 species from those of other countries, but they belong, for the 

 most part, to a group of genera found nowhere else. These are 

 the Achatinellince, of which there are seven arboreal genera 

 (Achatinella, Bulimella, Helicterella, Laminella, Partulina, New- 

 combia, and Auriculella), and three ground-genera (Carelia, Amas- 

 tra, and Leptachatina). 



Some of these genera are confined, in their distribution, to a 

 single island. The average range of each species is five or six 

 miles, while some are restricted to but one or two square miles, 

 and only a very few have the range of a whole island. 



The forest-region that covers one of the mountain -rauges of 

 Oahu is about forty miles in length and five or six miles in 

 breadth. This small territory furnishes about 175 species, repre- 

 sented by 700 or 800 varieties. The fall of rain on the north- 



