ISOLATION . 139 



exception (see below) there appears to be little really con- 

 vincing evidence that differences in rate of evolution are 

 determined by the environment. In this matter, however, 

 one positive example is probably worth several negative ones. 

 The exception referred to above is provided by island 

 races. We have already noted that endemism, though not 

 uniformly developed, is considerable. Not only are endemics 

 numerous, but they are sometimes of a peculiar type. Rensch 

 (1928, p. 174) has already noted that on small islands there 

 is a ' Neigung zu Excessiv-Bildungen in Grosse, Form und 

 Farbe.' We may note particularly : 



Dwarfing. Birds. (Rensch, I.e. pp. 174-5 ; Dwight, 1918, 

 p. 269.) 

 Tiger. (Pocock, 1929, p. 505.) 

 Mollusca. (Sturany, 1916, p. 137.) 

 Lizards. (Kammerer, 1926, p. 88.) 

 Giant forms. Lizards. (Kammerer, I.e.) 

 Mollusca. (Rensch, I.e.) 



(Not observed by other describers of 

 insular variation, e.g. Bristowe, Lundblad, 

 etc.) 

 Melanic forms. Reptiles. (Kammerer, I.e. ; Mertens, 1931, 



p. 205.) 

 Spiders. (Bristowe, 1929a, p. 164.) 

 Hydracarina. (Lundblad, 1930, p. 24.) 

 Mollusca. (Pelseneer, 1920, p. 561 ; 

 Aubertin, Ellis and Robson, 1931, p. 

 1049 5 Kammerer, I.e.) 

 Mammals. (Kammerer, I.e.) 



These rather striking consequences of life on islands require 

 further investigation. 



Another factor, viz. the numerical abundance of the species, 

 has been supposed (Darwin, 1884, pp. 42-3 ; Fisher and Ford, 

 1928 ; Ford, 1931, p. 100) to be important. Abundant 

 species are or tend to be more variable. A good example of 

 this is given by Fisher and Ford (I.e.) in the species of British 

 Noctuid moths. Greater variability will on the whole mean 

 quicker evolution. According to this idea evolution will 

 proceed by the fission of a few common, widespread variable 

 species, while the rarer, less variable species will become 



