86 FLEAS, FLUKES AND CUCKOOS 



the bristles and the shape of the seventh sternite. So far we know of no 

 explanation of the fact that one insect displays geographical variation 

 throughout its range and another, with the same host and the same 

 distribution, does not. However, it will be seen from the section on the 

 evolution of British bird fleas (p. 94) that these two species are not 

 very nearly related and C. hirundinis may be a much more recent acquisi- 

 tion of the house-martin than C.farreni. Time, and a certain degree of 

 isolation, is necessary for the estabhshment of subspecific differences in 

 a population of either fleas or birds. 



An exceedingly interesting case is that of the sand-martin and its 

 fleas. These birds are found in Europe and the United States and 

 specimens from both continents are indistinguishable. The martins as a 

 group are considered to have originated in the Old World and to have 

 spread to the New World in Pliocene times. On the grounds that the 

 sand-martin in the U.S.A. is not sub-specifically distinct from the 

 European bird, it is argued that it may well have invaded the New 

 World at a much later date, possibly in post-Pleistocene times. When 

 we come to consider its fleas a most surprising fact emerges. The 

 European sand-martin flea (C. styx) and the American sand-martin 

 flea (C. riparius) although very closely related are specifically distinct. 

 It is irresistible to suppose that the American flea is an oflfshoot of the 

 European flea, although it seems strange that it should have been more 

 aflfected by the new environment than the host. It must be remembered 

 that the main population has to over-winter in the nesting site whereas 

 the host seeks more congenial quarters farther south. Therefore one 

 can imagine that winter in the flea's new habitat might be strikingly 

 dififerent — say colder, or wetter, or more prolonged, than in the pre- 

 vious winter quarters. This might lead to a speedier evolution of the 

 parasite. 



Three important types of distribution are thus demonstrated by 

 the martin fleas. First, fleas which infest the host right across its 

 range of subspecies, but themselves show no geographical variation. 

 Secondly, fleas which show subspecific variation paralleling that of the 

 host. Thirdly, fleas which show either subspecific variation or specific 

 differentiation while the host itself remains unchanged. 



A fourth type of geographical variation is, however, illustrated by 

 fleas. D. gallinulae, as we have already shown, is not host-specific and 

 is found on a very great variety of birds. This species has diverged into 

 sub-species without any reference to the hosts. Thus in the Western 



