"GROUSE DISEASE " STRONGYLOSIS 237 



protozoan parasites into the body may suffice to cause serious 

 diseases owing to the rapid multiplication of the original germs. 

 In helminthic infections, as we have shown, the parasite does not 

 multiply inside the bird. Birds with few worms remain healthy. 

 The progress of the disease is correlated with the actual number 

 of parasites entering and surviving in the body. The more heavily 

 infected the food, the more heavily infected does the bird become. 

 The following facts connected with the growth of the parasite 

 outside the body of the Grouse emerged from the Inquiry, viz. : 



(1) that moisture is necessary for the development of the egg ; 



(2) that a minimal temperature of several degrees above freezing 

 point is essential not only for the development of the egg, but 

 also for the metamorphosis of the larva ; (3) that the larva 

 ascends the heather only after metamorphosis ; (4) that the 

 metamorphosed larvae are not killed by extreme cold ; (5) that 

 they may be killed by extreme drought. These facts afford 

 us some explanation of the disease being a fatal one in the 

 spring months. During the summer months many of the caecal 

 droppings must be dried by the sun and wind shortly after they 

 are passed, and the eggs thereby killed. The same agencies 

 must also desiccate beyond revival a large number of the 

 encysted larvae upon the heather. During the winter months, 

 however, this loss does not occur. Owing to the low temperature 

 and continual wet the eggs remain in a living but quiescent 

 condition. Even if an occasional spell of warmer weather 

 occurs, and the eggs develop into embryos, it would be necessary 

 that such period of high temperature should continue for at 

 least a fortnight to enable these embryos to become converted 

 into the active migrating larvae. The result probably is that 

 there accumulates upon the moors during the wh9le winter vast 

 numbers of undeveloped eggs and unmetamorphosed embryos. 

 The low temperature merely suspends their growth for the time 

 being. At the spring-time the minimal temperature rises 

 gradually to such a point as to allow the continuous develop- 

 ment of the eggs and embryos to and throughout metamorphosis. 

 The result is that at this period the accumulated result of 



