i 3 4 THE VARIATION OF ANIMALS IN NATURE 



of isolation are a very necessary supplement to any purely 

 topographical isolation. 



With this preliminary conclusion, we shall now return to 

 the main theme of the chapter and consider first topographical 

 isolation in somewhat greater detail, before passing on to the 

 intrinsic factors. The mere fact that most species have a more 

 or less extensive range automatically introduces a measure of 

 isolation between the more widely separated individuals. We 

 have already reviewed this question in Chapter IV, where we 

 came to the conclusion that, while habits and mode of repro- 

 duction may predispose a species to race-formation, the latter 

 process is not a very good index of the extent to which the 

 species-range is broken up by topographical barriers. Intrinsic 

 factors exert an important effect, which is at present largely 

 unpredictable. Possibly some of the anomalies might be ex- 

 plained away if we knew more of the minor migrations of 

 individuals that occur within the range of many species. 



An important point is that relatively slight barriers often 

 appear to be sufficient to determine the limits of races or 

 species. Thus in the Central Arabian desert, two races of 

 the rodent Meriones syrius (Cheesman and Hinton, 1924) 

 inhabit different stream valleys separated by only a mile of 

 bare limestone plateau. The intervening area is inhabited by 

 two quite distinct species. The habitat barrier is here much 

 sharper than would be normal in ordinary temperate regions. 

 Again, Wagner (1889, pp. 53-7) gives some instances, in various 

 groups of animals in N. Africa and Syria, of rivers acting as the 

 boundaries of races or species. In Chapter IV we have also 

 noted this in the case of squirrels (p. 116). 



Probably far more ecological knowledge of particular 

 species is required for a profitable discussion of topographical 

 isolation on continuous areas. It is possible, however, briefly 

 to review the problem of ' island-races,' since here the same 

 difficulties arise but in a more clear-cut form. 



When once a population has been cut off or immigrant 

 individuals have succeeded in reaching an isolated area, there 

 is much evidence in favour of the view that sooner or later the 

 fauna will undergo larger evolutionary changes. Probably 

 the oceanic islands, such as the Hawaiian or the Galapagos 

 groups, are the best examples of a high degree of geographical 

 isolation. Under these conditions it is well known that the 



