ISOLATION 141 



slow enough to allow considerable land-changes to occur 

 during the establishment of a race. In small animals geo- 

 graphical isolation becomes, on the whole, less important, for 

 even on continuous areas there are numerous ways in which 

 populations can be isolated from one another. While it might 

 be thought that none of these ways was sufficiently absolute 

 to allow permanent isolation to set in, the recent studies of 

 biological races point to another view. 



Methods of permanent isolation. 



These have been analysed by Robson (1928, pp. 122-33). 

 We recognise a primary division into two chief methods, each 

 of which may be subdivided. 



I. Indirect methods : 



(a) Seasonal occurrence. 



(b) Time of breeding. 



(c) General habitat. 



(d) Differences in breeding habitats. 



(e) Loss of means of dispersal. 



II. Direct methods : 



(1) Prevention of copulation. 



(a) Psychological or physiological. 



(i) Differences in specific recognition marks. 

 (ii) Differences in epigamic characters (scents, 

 courtship behaviour, secondary sexual 

 ornaments) . 



(b) Mechanical. 



(hi) By differences in the mechanical relations 

 of the copulatory apparatus. 



(2) Prevention of effective fertilisation. 



(c) By failure of the sperm to reach the egg. 



(d) By disharmonies in development, including and 



leading up to sterility in hybrids. 



In sedentary animals and aquatic animals with externa 

 fertilisation only I and II (2) can be effective. In motile 

 animals with internal fertilisation II (1) may also operate. 

 In this respect plants are in the position of sedentary animals. 



