172 



BIOLOGICAL PROBLEMS 



following table supplies evidence of this increase, 

 which is not based on simple opinion: 



But, it may be asked, did the birds produce a 

 noticeable effect upon the severity of the saw-fly 

 attack? The answer of a man of science to that 

 question must be cautious. We were attacking the 

 saw-fly by means of the birds, but there were many 

 other agencies working for us, as will be presently 

 related, over which we had no direct control, and, to 

 be quite frank, it cannot be proved to demonstration 

 that the birds were the primary cause of the happy 

 results obtained in 1912 when the woods were practi- 

 cally cleared of the saw-fly pest. It is significant, 

 however, that, whereas in 1912 the Thirlmere estates 

 were almost free from the saw-fly, the surrounding 

 districts of Cumberland and Westmorland were suf- 

 fering severely, and on the slopes of Skiddaw, within 

 sight of Helvellyn, no less than 3000 larches were 

 felled in 1913 on account of the attack of the previous 

 summer. So far back as the summer of 1911 the 

 improvement was manifest, as shown in the following 

 report of the local branch of the Royal Arboricultural 

 Society: 



"The larch plantations were found devastated by 

 the saw-fly, but at Thirlmere they were green and 

 vigorous. Here the Manchester Corporation have 

 waged successful war against the swarming cater- 



