142 PROCEEDINGS OP THE AMERICAN ACADEMY. 



diameter and transverse diameter, r is for Eddystone 0.845 ± .006, and 

 for the Irish Sea 0.784 ± .008. The sum of the probable errors is .014, 

 but the difference in the coefficients is .061, or four times the sum of the 

 probable errors. The results obtained for man are therefore confirmed 

 in these Mollusca. The coefficient of correlation of allied races is no 

 more constant than the means of their organs. 



2. Geographical Variation in Relation to Evolution. 



In a letter to Moritz Wagner, Darwin declares that he early appre- 

 ciated the importance of isolation in the origin of species, and the case of 

 the Galapagos Archipelago was one which led him to that appreciation. 

 And later students of island faunas have been struck by the change of 

 type in going from one constituent to another of any archipelago. The 

 literature on isolation as an evolutionary factor is indeed immense, but 

 only recently have biometric studies been directed to the variation of geo- 

 graphically separated form units. Some of the results of these studies 

 may be considered here. 



Geographic races are of two principal kinds : one results from isola- 

 tion combined with mutation ; such are found under essentially similar 

 conditions of life. The other kind results from isolation combined with 

 dissimilar conditions of life, and mutation may not occur. The first kind 

 is illustrated, apparently, in the Galapagos Islands. Each island has its 

 peculiar form of lizards and sparrows belonging to genera that are 

 spread over the whole archipelago. Yet the conditions of life on ad- 

 jacent islands, remnants as they probably are (Baur, 1892) of a single 

 island, must be remarkably similar — as similar as the parts of one of 

 the larger islands, in which, however, only one form occurs. Again, 

 the successive valleys of the island of Oahu, as we know from Gulick 

 (1873) and others, are remarkably alike in environmental conditions, 

 yet the snails of the genus Achatinella inhabiting them are very unlike. 

 Mayer (1902) has shown that the same thing is true for the partulas of 

 Tahiti. He says : — 



" It is prol)able that geographical isolation plays a most important part in 

 the formation of new species. If two valleys be adjacent, their snails are 

 closely related each to each, whereas the wider the separation between any 

 two valleys, the more distant the relationship between their snails. The 

 ridges between the valleys, being either barren or covered with vegetation 

 unsuitable to the snails, afford barriers over which the animals must find it 

 more or less difficult to pass. ... As far as the very limited observation of 

 the writer goes, there appears to be no diffv^reuce in the character of the 



