254 Biology in America 



divei'freiil will be llu'ir J'aiiiias and floras, liy ''separation" 

 in this sense is meant separation in time and environment 

 ratlun- tlian space. Thus the fauna of Australia and New 

 Zeahuid shows a vastly jrreater difference from that of th^ 

 Asiatic nuiinland, although separated therefrom by less than 

 2,000 miles, than does that of Japan from England, which are 

 about 8,000 miles distant from each other. By some biolo- 

 gists these differences are referred to the effect of isolation, 

 by others to the direct influence of environment (tempera- 

 ture, moisture, etc.), natural selection in either case exercising 

 the veto power or final control over the other factors. Prob- 

 ably all three factors are so closely inter-related in deter- 

 mining the final result in most cases that an exact analysis 

 of their relative roles is impossible. The inhabitants of cen- 

 tral Africa, separated from those of northern Africa by less 

 than 2,000 miles, differ more widely from each other, than do 

 those of North America and Siberia, separated by several 

 times that distance. But between the former intervenes the 

 wastes of the Sahara, impassable to most forms of life, while 

 the latter have an almost continuous land area between them, 

 broken only by the narrow Behring Strait, which freezes in 

 winter. Prior to the Miocene epoch a few million years ago, 

 which is comparatively recent, geologically speaking, tlie At- 

 lantic and Pacific Oceans were connected, where now ex- 

 tends the Isthmus of Panama. The elevation of the isthmus 

 has thus separated an originally single fauna into two, with 

 the result that many of the species on either side of the isth- 

 mus are represented by nearly related ones on the opposite 

 side, both doubtless derived from one ancestral form. 



In the instances already cited it is impossible to distin- 

 guish between the possible effects of environment, selection 

 and isolation ; in fact, it is very probable that all of them 

 have worked together in producing the final result. But in 

 the case of the land snails of the Hawaiian Islands, the two 

 former factors are seemingly ruled out, and isolation appears 

 to have been the only factor involved. The Hawaiian Islands 

 are a group of volcanic origin, the chief of which, Oahn, con- 

 sists of a long mountain ridge, rising to an elevation of 4,000 

 feet above the sea, from which extend inimerous lateral ridges, 

 with deep intervening valleys. Inhabiting these valleys are 

 800 or 1,000 different varieties of land snails (Achatinellidae), 

 over 200 of which the conchologists recognize as ''good spe- 

 cies." These snails feed upon the vegetation in the valleys 

 and seldom cross the high rocky ridges of the intervening 

 slopes. Each valley therefore has its own community of 

 snails, which is effectively isolated from neighboring com- 

 munities, but a few miles distant. As a result of such iso- 



