500 REV. J. T. GTJLICK ON DIVERSITY OF EVOLUTION" 



that the areas occupied by the different species and varieties of 

 that family will be more restricted than the areas occupied by the 

 species of other families that have greater opportunities for migra- 

 ting but the same tendency to variation. When we find that in 

 Europe and North America nearly every species of Helix occupies 

 an area many thousand times as large as the area occupied by 

 any Achatinella, we naturally ask whether the difference can be 

 accounted for by circumstances that limit the dispersion of the 

 latter, or whether the results are to be attributed to a stronger 

 tendency to variation. It is evident that to the forest species, 

 that live on trees found chiefly in the valleys, the mountain-ridges 

 separating the valleys must be partial barriers ; but the valleys 

 cannot be barriers to the species occupying the ridges, for the 

 ridges rising between the valleys are all spurs from the one 

 central range that forms the backbone of the island. In accord- 

 ance with these facts we find that the distances over which the 

 ridge species are distributed are usually somewhat greater than 

 those reached by the valley species. But even the ridge species 

 are limited in their distribution to very small areas. Few have a 

 range of territory more than six or eight miles in length and three 

 or four miles in breadth ; and many are restricted to half that 

 area. Though some of the groups of species are found both in 

 the valleys and on the ridges, so that no barriers intervene to 

 break the continuity of their intercourse, we still find them distri- 

 buted over small areas, and these areas again divided amongst 

 subordinate varieties. The streams that flow through these 

 valleys cannot serve in carrying the shells from one valley to 

 another ; but the separation from this cause can be no greater 

 than that which is experienced by mollusks inhabiting mountain- 

 valleys in other countries. It therefore appears that the limited 

 range of the species of this family receives but slight explanation 

 from the nature of the country. Neither can we suppose that 

 the power of locomotion in this family is so immeasurably below 

 that possessed by the Helices of Europe and America, and by the 

 Achatince of Africa, as to account for the excessive disproportion 

 in the areas occupied, as well as in the amount of divergence 

 between the types found in any locality and those found at given 

 distances. In Africa some of the species of Achatina have a range 

 of more than a thousand miles, while on the island of Oahu the 

 most widely diffused species of the arboreal genus Achatinella is 

 restricted to about ten miles, and the utmost limit gained by any 



