324 THE GROUSE IN HEALTH AND IN DISEASE 



definition and recognition of these conditions the health of the 

 moor directly depends. 



Briefly put, we have two factors common to all epidemic 

 diseases, the always present, occasionally harmful intruder, and 

 the host ; that is to say the Grouse, at times successfully 

 resistant, at times pathologically affected by the nematode 

 worm. In examining the action of these two variant factors 

 from the point of view of the moor-manager, all that is necessary 

 to ascertain is (1) with regard to the Strongyle what are the 

 predisposing causes which affect its occurrence in the Grouse's 

 caeca, in greater or less numbers with more or less harmful 

 consequences ? And (2) with regard to the Grouse what are 

 the predisposing conditions that tend to raise or lower the bird's 

 power of resistance to the ever present evil ? 



If we can get a clear conception of these two sets of contribu- 

 tory causes we can proceed with some confidence to investigate 

 measures put forward for the improvement of the health of 

 moors. 



Before considering the conditions affecting the degree of 

 infection of birds by the Strongyle worm, it is necessary to 

 refer to the life history of the parasite itself. 



As will be seen in chapter viii., this portion of the investiga- 

 tion has afforded the Committee no little difficulty ; the small 

 size of the worm, and the fact that a great part of its existence 

 is passed outside its host, have made it difficult to follow this 

 parasite through all its changes of form. The proverbial hunt 

 for a needle in a bundle of hay is simplicity itself compared to 

 the labour of detecting the larval nematode (so small as to be 

 visible only under a high-power microscope) in an acre of heather. 

 In addition to the difficulties arising from the small size of the 

 worm, the search was made more complicated by the presence 

 of other free-living nematodes, very easily mistaken for Tricho- 

 strongylus pergracilis. Some of these complete the whole cycle 

 of their life in the soil, and are never parasitic at all. 



With very few exceptions every Grouse has in its body 

 a varying number of Strongyle worms, of which the females 



