THE FAUNA OF THE MOOR 117 



out of condition, because of persistently bad weather, 

 or because of lack of food, or because the stock has 

 deteriorated through the survival of weakly birds that 

 the Golden Eagle and other enemies would have 

 sifted out if Man had not interfered. If there is a 

 deterioration of vigour and resisting power, then the 

 unpaying boarders inside the bird may begin to get 

 the upper hand and do serious damage. The cock- 

 Grouse seem to fall victims much more readily than 

 the hens, And then it is that we say " grouse disease." 

 This, at least, is one view of a difficult problem of the 

 hills. We are probably safe in saying that a removal 

 of the enemies that sift a wild race of animals by 

 cutting off the less vigorous and effective is likely to 

 be to the detriment of that race. We are probably safe 

 in saying that constitutional disease is practically 

 absent in wild nature, for if it appears it is promptly 

 nipped in the bud. We are probably safe in saying 

 that the customary parasites of an animal are not in 

 ordinary cases of deadly importance, but that they 

 may become so if the creature's natural vigour or its 

 ability to keep itself clean and fit is weakened. What 

 is apt to be very serious is when animals become 

 infected by a new kind of parasite, to which they are 

 utterly unaccustomed. This is what happens when 

 horses, for instance, are bitten by a tse-tse fly in 

 Tropical Africa, and thereby infected with an invisible 

 trypanosome, first cousin of the one which causes 

 sleeping sickness in Man. But it seems that various 

 wild Antelopes have trypanosomes in their blood 

 without being much the worse; they have established 

 a compromise with their parasite. 

 Another sound very characteristic of the moorland 



