ii4 THE VARIATION OF ANIMALS IN NATURE 



should evolve- where there are no very definite barriers, e.g. 

 races of E. and W. France. On the other hand, the existence 

 of several races in Switzerland, where mountain barriers are 

 numerous, suggests that isolation alone may account for the 

 changes observed. The identity of the English and W. French 

 races, however, is in disagreement with this view. Possibly 

 a survey of the hosts most commonly affected in different 

 areas might be important, though the variety of hosts appears 

 to be unusually great. 



5. Geographical variation in fishes. 



Examples are available of intense ' local race formation ' 

 in the sedentary ^oarces viviparus (Schmidt, 1918) and in 

 species, such as the Atlantic Cod (id. 1930), which have a 

 wider range. The latter is split up into ' a mosaic of popula- 

 tions,' each of which has a peculiar statistical facies in respect 

 of the two characters (number of vertebras and fin rays) 

 studied by Schmidt. 



6. Geographical races in squirrels and mouse-deer. 



It is well known that the squirrels of the Old World tropics 

 provide examples of some of the most extraordinary racial 

 complexes. The data are worth some consideration, since 

 they raise the question how far the variation of other 

 animals would prove equally refractory to schematic treat- 

 ment if more material were available. The races in squirrels 

 are largely separated by colour-pattern, differences in which 

 are sharply marked and easily studied. In such forms 

 as the smaller Muridae, where the study of each individual 

 requires a far more tedious technique and the characters 

 cannot be seized at a glance, a similar complexity might more 

 easily be masked. 



Evidence as to the African squirrels (Heliosciurus) may be 

 found in Ingoldby (1927) ; certain Burmese forms are dealt 

 with by Oldfield Thomas and Wroughton (1916), and Banks 

 (1931) discusses the Bornean races of Sciurus prevostii. The 

 last-named species has numerous races in Malaya, Sumatra 

 and Borneo. The latter island has about eight races, one of 

 which is also found on Sumatra or at least represented by a 

 closely similar form. Where the races overlap, intermediates 

 are found, almost certainly as a result of intercrossing. Some 



