A. J. Downing, Esq. - Dear Sir: I accede with pleasure to the request of your correspondent, Mr. Marshall, to state what grounds I have for subscribing to the theory advanced by Andrew Kkight, respecting the limited duration of varieties of plants.

Before I proceed, however, I wish to set Mr. Marshall right on one or two points; he says that I condemn propagation by extension in comparison with seedlings; if what I have said should convey that meaning to your readers generally, some apology is due from me for the imperfect manner in which I must have expressed myself, for I certainly do not condemn propagation by extension; on the contrary, I consider that when once a valuable variety of apple, etc, is obtained from seed, that multiplying it by divisions of the stem is a perfectly legitimate mode of propagation, and one that should be practiced so long as the individual variety retains its health and vigor; but beyond that period - when a variety exhibits manifest symptoms of declining vigor, and has become diseased and unproductive, through age, then I consider it should no longer be propagated by division, nor should seeds be saved from it with a view to raise new varieties, seeing that it is probable that the health of the seedling plants would be influenced to some extent by the unhealthy and degenerate condition of the parent tree.

I may further remark that I am not prepared to prove "that trees and plants propagated by extension, do produce degenerate fruit from that very cause, and that alone." I am not aware of having said anything in the paper referred to, which will justify the conclusion that I entertain any such notions. In the matter of the potato, I stated what I believe to be the exciting and chief predisposing causes of the blight; I consider the potato to be in a condition different from that of any other cultivated plant; that considered in the mass, or as a species, it is hereditarily diseased; believing this, and knowing that it had been observed in the case of the pear, that seedlings raised from old, nearly worn out varieties, proved, as might reasonably have been expected, unhealthy, and liable to disease also; and knowing, moreover, that many varieties of potatoes recently obtained from seed, were subject to dry rot, and as much injured by the blight, as older varieties, I concluded that the best way to get rid of the hereditary taint, was to persevere in raising a succession of seedlings, with improved culture, selecting the strongest and healthiest plants each year, to be the parents of a fresh generation of seedlings in the year following.

If seeds were saved from a healthy variety of fruit tree, or other plant, when in the prime of its existence, although the plant it was saved from had been propagated by extension, I know of no reason why the progeny should not be perfectly health)'. Nor is there any reason for believing otherwise, than that a species of plant whose varieties are propagated by extension, may not be continued equally healthy and vigorous forever, providing the successive generations of seedling varieties were always raised from seeds taken from plants when in a healthy and vigorous condition.

Now for my "hobby." Perhaps it may be well at the outset to stale briefly the nature of the hypothesis we are about to consider. It is this. Vegetable life, like animal life,

1 has its fixed periods of duration. A seedling apple tree, for instance, has its periods of youth, maturity, and old age. All cuttings taken from this seedling apple tree and graftnewal or reproduction, as by seed. They will all exhibit, (if we may so speak,) a sympathetic state of health, making, of course, due allowance for the action of adventitious circumstances; and although some plants, if placed in unusually favorable circumstances, may out-live the parent tree, yet there will come a time "beyond which the debility incident to old age, cannot be stimulated;" all plants of the variety will consequently become diseased and worthless.

A knowledge of this hypothesis is of importance to all cultivators, because if it is well founded, it shows to us the hopelessness of striving against nature, by persevering in the cultivation of varieties of plants when they have become aged and unhealthy, and no longer able to make an adequate return for the labor and attention bestowed on them. It shows also, the necessity of keeping up a succession of varieties from seed, and that it is an important matter to consider the age and health of a variety, when our object is to obtain from seed, new, improved, and healthy varieties.

In order that Mr. Knight's hypothesis may be better understood, generally, and that I may be better able to prove what substantial grounds it rests upon, I have thought it advisable to direct attention to the most recent and elaborate attack on it - viz: two articles of Prof. Lindley, in the Gardeners' Chronicle of December 13 and 20,1845; for the value of a theory is made manifest not only by the accuracy of the facts on which it is based, or the soundness of the reasoning by which it is supported, but by the fallacy of the arguments by which it is assailed, more especially if the assailant is a man of acknowledged ability, and acquainted with the subjects on which he writes.

The opinion of systematic botanists generally, on horticultural matters. - I mean men whose lives have been chiefly devoted to the classification and description of plants, does not seem, so far as my experience extends, to be entitled to any very great weight. Lindley, however, is an exception - the son and brother of a nurseryman, Vice-Secretary of the Horticultural Society Of London, and Editor of the Gardeners' Chronicle, he has enjoyed opportunities of obtaining a more thorough knowledge of the details of practice, and the history of cultivated plants, than any other man now living who possesses anything like the same amount of theoretical knowledge; and when, with these advantages, we consider that he wields the pen of a ready and plausible writer, it may be presumed that if any man were capable, by means of facts at present known, of proving that Mr. Knight's views on this point are erroneous, Lindley would assuredly be that man.