8 THE GROUSE DISEASE chap. 



they create a less resisting, more susceptible material 

 for the carrying out of -infection. The more adverse 

 the season — inclemency of the weather, frost, etc. — 

 the more sparse or inferior the natural food, the less 

 hardy and strong are the birds when the time arrives 

 which is most favourable for the development of the 

 disease, and therefore the more easily do they take 

 infection. This is well understood as applicable to 

 other infectious disorders. A strong individual, not 

 weakened by bad and insufficient food or by bad or 

 inclement weather, will resist the infection much more 

 easily than one previously brought down in general 

 health and strength. Suppose there are, on a given 

 moor, a large number of birds in this weakened state, 

 and suppose there exists on this moor the active 

 cause of the disease, it stands to reason that, there 

 being present a good many birds of no great resisting 

 power, one or the other of them will take the infec- 

 tion, from these others will take it, and so on, so 

 that, after a short time, a large number of foci will 

 thus be started acting not only on the remainder of 

 the weaker but also on the stronger birds. But sup- 

 posing, on the other hand, the birds in early spring 

 are well and strong ; though the active cause of the 

 disease be present on this moor, yet it will not easily 

 find, as it were, a susceptible individual, and conse- 

 quently the disease will show itself either only in 



