List of the Insect Fauna of the County. 109 



'*It may be observed, however, that these great differences 

 are marked m outer shape and construction much more than 

 in minute structure. As, in human pathology, there are 

 certain general characters and degrees of likeness in all 

 inflammatory products, however differently they may be 

 constructed — in pustules, vesicles, thickenings, opacities, 

 adhesions, scars, fibroid and other changes ; so, in galls, 

 there are certain likenesses in minute structures, even among 

 those that are, in their construction, size, and outer shape, 

 most unlike. 



"It may be well to learn from this a lesson on the imper- 

 fection of our methods of minute research. As we cannot 

 doubt that tlie differences in outer shape and method of 

 construction of the products of specific diseases are associated 

 with differences of chemical composition and ultimately 

 minute structure, so it must be in those yet greater differences 

 on which we frame our distinctions of species in all living 

 nature. The coarse, visible, and tangible distinctions may 

 be well marked ; the really material differences with which 

 these are associated, and to which probably they are due, are 

 beyond our reach. 



" Again, in the study of specific diseases in ourselves, we see 

 many variations due to the differences in the parts, or 

 even in the persons afl'ected with them. In the study of 

 galls, similar variations may be seen. As a general rule, 

 each gall-insect lays its eggs in one part of one plant — as the 

 leaf, leaf-stalk, bud, fruit, or root of this or that species; but 

 if — as rarely happens — one lays in different parts of the same 

 plant, there is usually a very close agreement in the characters 

 of the resultant galls. A few exceptions to this rule are 

 known, one of them being in the very different galls produced 

 on the roots and on the leaves of vines by Phylloxera vastatrix; 

 but the rule is generally observed, and accords with the fact 

 of certain features of general likeness being observed in the 

 products of our several specific diseases, wherever they may 

 be seated. 



"When the same insect lays in similar parts of different 

 plants, the galls may be all similar ; but I believe that they 

 more usually are different, and that their differences are such 

 as bring them severally nearer to the distinctive characters 

 of the plants on which they grow ; just as, in ourselves, a 

 specific disease may be modified by the personal conditions of 

 each patient. 



"In similar analogy, the differences are yet greater when 

 the eggs are laid in different parts of different plants. 



