CHAP. VII GENERAL CONSIDERA TIONS 59 



lung and liver of grouse dead of the grouse disease 

 is the causa causans of the disease. It will no doubt 

 be argued that the chain of evidence as to this par- 

 ticular microbe being the cause of the grouse disease 

 is in so far incomplete as no infection of normal 

 grouse has been carried out with it. The force of 

 this argument I fully admit, and I have endeavoured 

 to meet it ; but owing to the difficulty which I en- 

 countered in getting permission to inoculate with 

 the microbe young grouse on the moors on which 

 no grouse disease prevails, all my endeavours have 

 failed. It is clear that such experiments must be 

 carried out on moors on which no grouse disease 

 exists, for if the experiments were made in a 

 locality where grouse disease occurs, the objection 

 could with justice be raised that accidental or 

 natural infection of the experimented grouse was 

 not excluded. I do not think that it would be 

 very difficult to separate a few young grouse 

 — although grouse do not easily bear seclusion 

 in a cage — before they go for cover, and to in- 

 oculate them. But this would require my going to 

 and staying at the moors, and getting permission to 

 do it, an expensive experiment which I am not in 

 a position to carry out. But I tried to meet the 

 difficulty in a different way, viz. by getting eggs of 

 grouse and having them hatched by hens. With 



