LETTER XIX. 



211 



generation indefinitely, and so be perpetuated in the 

 family. 



8. And this observation we may, I think, apply to 

 the Potato and its disease," and likewise to the 

 Vine, the Hop, the Peach, and their diseases." 

 Heretofore, the diseases which have so widely pre- 

 vailed — in fact epidemically — among the plants in 

 question, and which have proved so destructive to 

 them and so disastrous in their issues to the people 

 of many countries, have eluded all attempts to ascer- 

 tain either their real nature or their exciting causes, 

 as well as baffled the skill of man either to prevent or 

 cure them. And our main hope, it appears to me, 

 must lie in the evanescent character of the diseases 

 themselves and the restriction of them to individual 

 generations only, of the race to which the plants re- 

 spectively belong, 



9. One qualifying observation I must needs add in 

 regard to trees, which is, that it may be a question, 

 even in the point of view just adverted to, whether 

 the timber produced by diseased plants, overlying, on 

 the one side, and overlaid by, on the other, the sound 

 timber of pre-existing and succeeding healthy plants, 

 may not affect the quality of the entire " Timber- 

 stack." — May not the seeds of the ^'dry-rot" in 

 timber, for example, attach themselves primarily to 

 particular rings or cylinders of the tree, and have 

 their origin, not so much — or at least so exclusively, 



