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 propagated 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 ingrafted; I believe those grafted trees will continue to grow for ten or twelve years, before they bear fruit, almost as long as seedling trees, that is they will require as much time, as those new shoots from the lopped trunk would require, before they produce fruit. It should thence be inquired, when grafted fruit trees are purchased, whether the scions were taken from bearing branches, or from the young shoots of a lopped trunk; as the latter, I believe, are generally sold, as they appear stronger plants. This greater similitude of the progeny to the parent in solitary reproduction must certainly make them more liable to hereditary diseases, if such have been acquired by the parent from unfriendly climate or bad nourishment, or accidental injury. In respect to the sexual progeny of vegetables it has long been thought, that a change of seed or of situation is in process of time necessary to prevent their degeneracy; but it is now believed, that it is only changing for seed of a superior quality, that will better the product. At the same time it may be probably useful occasionally to intermix seeds from different situations together; as the anther-dust is liable to pass from one plant to another in its vicinity; and by these means the new seeds or plants may be amended, like the marriages of animals into different families.