A. J. Downing, Esq. - Dear Sir: Pardon me for again touching a subject on which there has been so much speculation, without really advancing new facts, or shedding additional light to aid in removing the mist in which it seems enveloped.

I am led to the subject at this time to correct an error into which Prof. Turner of 111., has fallen, in his article in your last number, (June.) I do not mean to review or criticise the consistency of his writings on this subject, but leave that for him to do at his leisure. I shall confine myself to his last mistake, in supposing that he had discovered a new insect, which he thinks is the cause of the mischief. He seems truly alarmed at the discovery and the prospect before him. I do not wonder that he is, for if his suspicions were well founded, it would indeed present a hapless despair, which he might well denominate the " Pear Devil." Well, for one of a partial and fanciful turn of mind this is certainly a subject on which to display its powers of imagination. He seems really in an unhappy state of mind, and one might almost infer that he is a believer in total depravity, and the idea that all animate and inanimate matter is but one mass of "living atoms," preying on each other's miseries; the only remedy for which is to purge or poison them to death.

I hope on a little further acquaintance with the new (to him) form of blight, he will not find it so bad; and that there is still much to comfort and reward him for his labor.

Let us then come to matter of fact, and see how that stands. That he discovered the existence of insects new to him, and described them as he saw them, I presume no one would have required the confirmatory statement of a witness. But, that they have escaped the observation of others, or that they are the cause of the blight, is quite a different matter. If Prof. T. will turn to "Harris' Treatise on Insects," under the head of " Bark-lice," he will find a full description of the scale insect which he found on the branches of his trees. They are of very common occurrence among young fruit trees, especially in the nursery, and yet this is not the place we find the blight, of which we are speaking. I have seen young trees here, and at the north, where blight is but little known, literally covered with them, and though injurious to the growth and health of the plant, I have never seen the evidence of their connection with blight. The little fellows which resemble the " sow bug," which he saw "running about between the fibres of the bark," were long since introduced to the members of our Horticultural Society, in their researches, in common with others which are usually found about the vitiated parts of blight.

I do not understand if Prof. T. means to convey the idea that all the insects he saw in connection with the blight, are the emanations of the eggs under the scales on the branches of the trees. I wish he had been less ambiguous about it.

I have had occasion to deplore the destruction of my trees, giving me much reason for close and critical observation, to detect, if possible, the cause, and a remedy for its destructive effects. I have not been idle in improving the opportunities thus afforded. A part of these researches the public are already in possession of through your Journal, vol. 2, p. 328 and 436, and others. I have seen no reason for abandoning the views there presented, but subsequent observations have confirmed their soundness. It is not my purpose to reiterate them here; those who feel an interest in them can refer to them; nor do I hope to add much that is newt.

It is of little consequence to the public, or ourselves, whether there is one or twenty sorts of blight, so long as we are. grovelling in the dark about the first principles of its origin. My experience has entirely removed from my mind the idea that the blight is caused by insects, and equally satisfied my judgment of its real cause. This I do not hesitate to say, is altogether external - originating with the rapid changes of heat and cold, stimulating and suddenly checking the active motions of vegetation; and that insects have no more to do in producing it, than they have in causing bilious fever in the human system, though death may ensue, and insects, in either case, be attracted by the disease, and the abnormal condition thus produced.

I take it there is a strong analogy between vegetable and animal life and existence. That neither the one or the other can be transported from clime to clime, and subjected to unused fare, without endangering health; or can a system of crossing, having in view a different object than that of hardiness and long life, be pursued without the strong probability of sacrificing the one at the expense of the other. This is truly our position, whether applied to the pear, cherry, rose, or other exotic. We have contented ourselves with importing from other countries their enfeebled stocks. These will suffer just in proportion to their capabilities to resist the new influences under which they are brought. It matters not whether it be the frost of winter, or the scorching rays of a mid-summer's sun, or both combined; it is still blight, and conclusively proves the want of hardiness.in the tree or plant, for our climate. It does not help us, that we import our seed, and from this produce some new and good sorts - they are as liable to be constitutionally defective, as if grown in Belgium or France, and then imported; it still remains the same enfeebled progeny.

A query may here arise, what shall we do? Shall we give them up in despair? By no means. We must begin at home; we must select our seed from trees, the structure of whose wood has given evidence of its power to resist this atmospheric influence. We have such - the Seckel, with many others of inferior fruit, but perfectly hardy trees, which my long and severe trial has proven. These we must fertilize with each other, and from their seed produce a new race of superior fruit, and hardiness of tree. Let us, then, not set down in hopeless despair, but go cheerfully to the work, enjoying a bright future as though it were present.

Hoping that what I have said in reference to the Professor's mistake, will not be constru ed as disrespectful, I remain very respectfully yours, A. H. Ernst.

Spring.Garden, Cincinnati, June 12, 1852.