ISOLATION 137 



Mollusca. No endemics x on the Scilly Isles (Richards 

 and Robson, 1926). Probably no endemics on the 

 Hebridean Islands (Robson, MS.). This may be 

 contrasted with the high degree of endemism in the 

 mammals. 



A somewhat similar phenomenon is the capricious occur- 

 rence of endemism in archipelagoes. We have already given 

 a few examples (e.g. mouse-deer, p. 116). Simpson (1929), 

 in his study of the species of Liguus (land snails) on the Florida 

 Keys, finds that they are broken up into numerous varieties, 

 but that there is no regular localisation on particular keys 

 (contrast with ' ridge ' forms of Partula (Crampton) ). A 

 given variety may occur on several keys, and a given key 

 may have only one or else several varieties. There appears 

 to be no obvious correlation between topographical isolation 

 and varietal differentiation. 



Similarly Riley (1929) finds that the birds of the Sumatran 

 Islands are on the whole more differentiated on the remote 

 islands than on the less remote. But this is not invariable, and 

 in the W. Sumatran Islands the relation between differentiation 

 and spatial separation is not nearly so obvious (cf. Robson, 

 1928, p. 139 (Hebridean mammals) ; also Aubertin, Ellis and 

 Robson, 1 93 1 (colonies of Cochlicella acuta) ). 



We are led, therefore, to inquire as to the circumstances 

 in which some species change or remain stable ; and, secondly, 

 as to whether numerous smaller factors tending to produce isola- 

 tion on a small scale are not just as important as the high degree 

 of isolation produced by marked geographical separation. 



The relative stability of some species and the high degree 

 of variability in others provide one of the most curious and 

 baffling problems in biology (cf p. 106, Chapter IV). It 

 is remarkable to what an extent certain species of a genus 

 may vary, when others are quite constant. The same differ- 

 ences are found in the frequency with which geographical 

 races are formed. It might be supposed that such differences 

 in variability depended on whether a species was exposed to 

 constant and homogeneous or varying and heterogeneous 

 conditions. But in fact all who have analysed such cases 

 agree that no such detailed relation can be found. With one 



1 i.e. definite subspecies. 



