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The feeble births acquired diseases chase,As all the families both of plants and animals appear in a state of perpetual improvement or degeneracy, it becomes a subject of importance to detect the causes of these mutations.
Till Death extinguish the degenerate race. CANTO II. l. 165.
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 of 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, {44} 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 a 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.
As the sexual progeny of vegetables are thus less liable to hereditary diseases than the solitary progenies; so it is reasonable to conclude, that the sexual progenies of animals may be less liable to hereditary diseases, if the marriages are into different families, than if into the same family; this has long been supposed to be true, by those who breed animals for sale; since if the male and female be of different temperaments, as these are extremes of the animal system, they may counteract each other; and certainly where both parents are of families, which are afflicted with the same hereditary disease, it is more likely to descend to their posterity.
The hereditary diseases of this country have many of them been the consequence of drinking much fermented or spirituous liquor; as the gout always, most kinds of dropsy, and, I believe, epilepsy, and insanity. But another material, which is liable to produce diseases in its immoderate use, I believe to be common salt; the sea-scurvy is evidently caused by it in long voyages; and I suspect the scrofula, and consumption, to arise in the young progeny from the debility of {45} the lymphatic and venous absorption produced in the parent by this innutritious fossile stimulus. The petechiæ and vibices in the sea-scurvy and occasional hæmorrhages evince the defect of venous absorption; the occasional hæmoptoe at the commencement of pulmonary consumption, seems also to arise from defect of venous absorption; and the scrofula, which arises from the inactivity of the lymphatic absorbent system, frequently exists along with pulmonary as well as with mesenteric consumption. A tendency to these diseases is certainly hereditary, though perhaps not the diseases themselves; thus a less quantity of ale, cyder, wine, or spirit, will induce the gout and dropsy in those constitutions, whose parents have been intemperate in the use of those liquors; as I have more than once had occasion to observe.
Finally the art to improve the sexual progeny of either vegetables or animals must consist in choosing the most perfect of both sexes, that is the most beautiful in respect to the body, and the most ingenious in respect to the mind; but where one sex is given, whether male or female, to improve a progeny from that person may consist in choosing a partner of a contrary temperament.
As many families become gradually extinct by hereditary diseases, as by scrofula, consumption, epilepsy, mania, it is often hazardous to marry an heiress, as she is not unfrequently the last of a diseased family.