OTSTEE-SHELL BAKK-L0TJ8E. 31 



of which the scale is composed are so distinct and contrasted in 

 this elegant species, tliat the investigation of it is much more prac- 

 ticable and satisfactory. We must therefore refer those who are 

 curious in these abstruse and controverted branches of the subject, 

 to the history of that species, at the end of this report. 



We pass to a more practical question, whether the Oyster-shell 

 Bark -louse flourishes best upon a healthy or a debilitated tree. 

 This question also has its difficulties. For if the Bark-louse does 

 not find the tree sickly, it makes it so, and as the two things al- 

 ways go together, it leaves an uncertainty which is the cause and 

 which the effect. It is like the old question of the ague and the 

 quinine: which it is that has damaged the constitutions of so many 

 Western people. The popular hue-and-cry is against the quinine, 

 which is a pretty good illustration of the danger of keeping bad 

 company. We take the quinine only when we have the ague, 

 and the two things becoming confounded in our experience, we 

 perversely conchide that the disease is harmless and that the heal- 

 ing medicine does all the mischief. 



That an insect, that lives by imbibing the sap of a tree, should 

 flourish better upon a half dead and dried up tree than upon a 

 thrifty and succulent one, is, on the face of it, extremely improb- 

 able. The conclusion to which I have come, both from reason and 

 observation, is that if bark-lice get foot-hold upon a tree which is 

 congenial to them, they will multiply and impoverish it, however 

 healthy it may be at the time of attack, or however well the tree 

 may be cultivated. 



And this leads us to another question of considerable practical 

 importance, and this is, whether the Oyster-shell Bark-louse exhib- 

 its any preference or exercises any selection between the different 

 varieties of apple tree. That this is the case is, I believe the gen- 

 eral opinion, and I am perfectly satisfied of it from my own ob- 

 servations. I saw the truth of this most satisfactorily illustrated 

 in the orchard of Mr. Robson, of Galena. Here were trees some 

 of which must be presumed to have been congenial, and others 

 uncongenial to the insect, intermixed with the same inclosure, 

 and the curious spectacle was exhibited of trees standing side by 

 side, or alternating with each other, some of which were almost 

 covered with scales, and others nearly or quite clear. These trees 

 were so similarly situated with respect to all outside agencies that 



