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of which, the scale is composed are so distinct and contrasted in 
this elegant species, that 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 ot 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 conclude 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. Bobson, of Galena. Here were trees some 
of which must be presumed to have been congenial, and others 
uncongenial todheJnsect, intermixed with the same inclosure, 
and the curious spectacle was exhibited ot 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 
