n6 THE VARIATION OF ANIMALS IN NATURE 



of the races, however, are sharply isolated from one another 

 by rivers. Oldfield Thomas and Wroughton also note the 

 importance of rivers as barriers to the Burmese forms. Banks, 

 further, finds that individual variation within the races is 

 extreme and appears partly to produce forms which might be 

 called races were it not that they do not form definite popu- 

 lations. Thus in S. prevostii borneensis, according to Banks 

 (I.e. p. 1336) — ' No two specimens are alike, and the 

 variation is endless.' Both colour and pattern are affected, 

 and Banks shows it is very difficult to correlate the characters 

 of the races with any known feature of the environment. 

 Apart from one mountain race, most of them appear to live 

 under very similar conditions, the island being tropical through- 

 out. It is also interesting that certain races appear to have 

 a discontinuous distribution, such as has already been noted 

 in the flea Ctenophthalmus agyrtes. A similar example of dis- 

 continuous geographical groups is found in the Carrion Crow 

 (Kirkman and Jourdain, 1930, p. 2). An E. Siberian form 

 of this species is separated from the main area of the species 

 by the whole distributional area of the Hooded Crow. It 

 cannot, of course, be proved without elaborate genetic experi- 

 ments that apparently similar forms are really identical, but 

 the formation of similar races in different areas within a larger 

 patch of uniform conditions is strongly suggestive of the 

 convergent establishment of the same chance combinations 

 of genetic factors. It may be mentioned that Bequaert (1931) 

 has shown that the geographical race of the Hornet (Vespa 

 crabro) inhabiting the British Isles, resembles a Chinese race 

 far more closely than it does the adjacent continental form. 



In the African Heliosciurus, Ingoldby has shown that similar 

 races tend to be found on each side of the equator, with races 

 of a different type lying between them. Here there is a greater 

 possibility of a direct environmental effect and, according to 

 this author, the races in two localities with identical ecological 

 conditions are the same. It is not difficult, however, to find 

 instances where there is no obvious correlation with the 

 environment ; in fact such correlation appears to be the excep- 

 tion rather than the rule. Thus Miller's study of the Malayan 

 mouse-deer (Tragulus) (1909) shows that numerous races have 

 been developed under conditions as nearly uniform as possible. 

 In this genus races are more often developed on the smaller 



