ISOLATION 147 



relatively uniform and are not separable into subspecific units, 

 while other species which breed over a large area are much less 

 homogeneous, being formed of a number of separate strains. 

 From the point of view of isolation, it is difficult to distinguish 

 between the action of geographical barriers and of differences 

 in migratory instincts. 



I (e) . Loss of means of dispersal. 



The examples cited in the previous paragraph lead us 

 naturally to consider animals in which the power to migrate 

 has been lost. Our ignorance of this matter is much greater 

 than would appear at first sight. The high percentage of 

 endemism on islands is well known, as is the tendency for 

 island forms of winged species to be apterous. Evidently, if 

 the species had not been winged originally their chances of 

 reaching an oceanic island would have been small. Once an 

 island has been reached, loss of powers of dispersal will aid the 

 formation of local colonies, though it will not aid in isolation 

 from fresh immigrants. We need not consider at this point 

 the theories that have been put forward to account for the 

 winglessness of island species, but, however produced, apterism 

 will tend to multiply the numbers of endemic species on an 

 island. At the same time very numerous examples of complete 

 or nearly complete loss of wings are known from continental 

 areas. In the beetles these facts have been summarised by 

 Jackson (1928), and for Diptera by Bezzi (1916, 1922). The 

 former author, working on the weevils of the genus Sitona, 

 found short- winged, long-winged, and dimorphic species. 

 Wherever the power of flight has been lost we might expect 

 some degree of isolation to arise between colonies that pre- 

 viously were able to interbreed, if only because the ordinary 

 habitat of the animal is not likely to be continuous. But we 

 have to be very careful not to assume that the species with 

 apparently the best means of dispersal are necessarily the most 

 active species in getting about. Thus Richards (1926) points 

 out that the wingless beetle Helops striatus is one of the first 

 insects to re-invade heaths after fires. The wide range of 

 many other wingless forms suggests that detailed knowledge of 

 actual methods and powers of dispersal is necessary before we 

 can assume very much about their significance in isolation. 

 The loss of eyes in cave insects is a parallel phenomenon. 



