43 



4 



ADDITIONAL NOTES. XL 



HEREDITARY DISEASES. 



The feeble births acquired diseases chase, 



Till Death extinguish the degenerate race. CANTO II. 1. 165. 



As all the families both of plants and animals appear in a state 

 of perpetual improvement or degeneracy, it becomes a subject of im- 

 portance to detect the causes of these mutations. 



The insects, which are not propagated by sexual intercourse, are 

 so few or so small, that no observations have been made on their 

 diseases ; but hereditary diseases are believed more to affect the 

 offspring of solitary than of sexual generation in respect to vegetables; 

 as those fruit trees, which have for more than a century been propa- 

 gated only by ingrafting, and not from seeds, have been observed by 

 Mr. Knight to be at this time so liable to canker, as not to be worth 

 cultivation. From the same cause I suspect the degeneracy of some 

 potatoes and of some strawberries to have arisen ; where the curled 

 leaf has appeared in the former, and barren flowers in the latter. 



This may arise from the progeny by solitary reproduction so much 

 more exactly resembling the parent, as is well seen in grafted trees 

 compared with seedling ones; the fruit of the former always resembling 

 that of the parent tree, but not so of the latter. The grafted scion 

 also accords with the branch of the tree from whence it was taken, in 

 the time of its bearing fruit; for if a scion be taken from- a bearing 

 branch of a pear or apple tree, I believe, it will produce fruit even 

 the next year, or that succeeding; that is, in the same time that it 

 would have produced fruit, if it had continued growing on the parent 

 tree; but if the parent pear or apple tree has been cut down or headed, 

 and scions are then taken from the young shoots of the stem, and in- 

 grafted; I believe those grafted trees will continue to grow for ten or 

 twelve years, before they bear fruit, almost as long as seedling trees, 



