CHAPTER V 



ISOLATION 



The importance of isolation in evolution was first strongly 

 insisted on by M. Wagner (cf. summary of his work, 1889). 

 Darwin also allowed its influence to be considerable, as, for 

 instance, in the production of island races. Both these authors 

 regarded adaptation to the local conditions as of fully equal 

 importance (cf. Wagner, I.e. p. 401). In Chapter I it was 

 indicated that isolation may be regarded as playing two 

 opposing roles in the process of group-formation, viz. the 

 maintenance of the identity of groups and the splitting up of 

 large groups into smaller ones. In the present chapter this 

 matter is considered more fully. 



The more general problems of geographical distribution 

 need not be given special attention. They have been dis- 

 cussed at length in many works wholly devoted to the subject. 

 For the same reason actual dispersal mechanisms are only of 

 secondary interest. These also have been much discussed, 

 but well-authenticated data are somewhat meagre and scarcely 

 sufficient to enable us to formulate any general relation 

 between powers of dispersal and race-formation. Allusion 

 has already been made to this difficulty in Chapter IV 

 (p. 104), and it may be added that any such relation 

 might be obscured by innate tendencies to race-formation 

 which appear to be independent of dispersal. Two main 

 types of isolation itself may be recognised. Geographical or 

 topographical isolation is operative when two populations are 

 separated by uninhabitable country. Sections of a species 

 isolated by such a barrier would, for some time after their 

 separation, be able to interbreed if they could be carried 

 across the barrier. Isolation of this kind is temporary, since 

 without changes in the animal itself it is always liable to break 

 down as a result of modification of the barriers themselves 

 (e.g. movements in the earth's surface). Jordan (1896, p. 442) 



