4 FLEAS, FLUKES AND CUCKOOS 



of the human blood fluke, which was a serious plague throughout the 

 Middle East, could easily have been elucidated in this country and 

 much time and effort saved if we had made use of the similar type of 

 worm found here in the veins of ducks and gulls. From the utilitarian 

 biologist's point of view it is difficult to over-estimate the importance 

 of studying parasites. Birds which harbour many species closely 

 related to those normally infesting man render us a silent but in- 

 estimable service by their sad experiences. Most of the successful anti- 

 malaria drugs are first tried out on canaries. 



From the naturalist's point of view, which is necessarily a rather 

 different one, parasites are equally important. Broadly speaking, the 

 public are no longer interested in evolution. The man in the street, 

 who has survived two world wars, together with mustard gas and the 

 atomic bomb, now accepts the suggestion that he is descended from the 

 apes without either indignation or surprise. Evolution has, however, 

 remained the lodestar of our generation of naturalists, and parasites are, 

 perhaps, the organisms in which evolution is most obvious. Their mode 

 of life has imposed certain definite morphological and physiological 

 modifications upon them — a sort of gigantic secondary experiment in 

 evolution, which, if properly studied, must prove profoundly illuminat- 

 ing. Moreover parasites act as pointers and guides to the evolution and 

 relationship of their hosts. Between these two an eternal and curious 

 struggle is in progress. The host's reactions are wholly hostile but the 

 parasite is forced to adopt a compromise. It has to restrict its activities 

 in such a way that it does not immediately endanger the host's life and 

 thus jeopardise its own food supply and chances of reproduction. 

 Parasites which neither stimulate the host to violent reactions nor 

 inflict upon it serious permanent injury are said to be "well adapted" 

 to their mode of life. 



There are in nature certain associations in which the organisms 

 concerned suffer no ill effects and, on the contrary, are assured either 

 unilateral or mutual benefits. These relationships, which are known as 

 commensalism, symbiosis, and phoresy (see p. i8) may represent 

 transitional stages in the development of the parasitic habit. Some 

 hold the view that they precede parasitism. Others, with a more 

 ideahstic outlook, consider that adaptation has here evolved beyond 

 the parasitic relationship, with the ehmination of harmful effects and 

 a gradual substitution of mutual benefit. Whatever the truth may 

 be it is clear that a study of the borderhne associations is of consider- 



