The life of an animal is marked by three distinct stages, progressive, conservative, and declining. In youth, the greatest amount of food is assimilated; the body increases ra-piply in size, and the limbs are supple; in middle age, little more food is appropriated than is required for the repair, or solidifying of the frame; while in the decline of life, an animal gradually becomes meagre and diminishes in size - the quantity of food assimilated is not equal to the waste of the body. " It is certain that the productive powers of a variety of the potato, is in proportion to its youth." (Lindley).

It is certain, that owing to a progressive decline of the vital powers, possibly to the less efficient state of the feeding organs, the plant is unequal to the task of absorbing and es-sknUatiog the same amount of food as in its youth; herein there is obviously a great similarity between plants and animals. I believe it is equally certain, that a structural change does gradually occur in plants, as well as in animals, when an individual declines in vigor, which change cannot be attributed solely to the action of external agents.

I may again quote the observation of the Editor of the Irish Farmers' Magazine, respecting the gradual changes induced by age, in the quality and productiveness of a variety of potato. " In a few years after a variety has been raised from seed, it arrives at its greatest degree of productiveness; then it continues annually, for a number of years, to decrease in productiveness, but to become more valuable for food, being more farinaceous, or as it is termed drier; afterwards it begins to lose this quality, also, and rapidly to decline, until in a few years more, it is utterly useless." Similar observations occur in the tenth vol. of the Quarterly Journal of Agriculture. In vigorous growing, productive varieties of the potato, yielding at first coarse grained tubers, so full of fluid sap as only to be fit for cattle, this progressire change in the quality, and consequently in the composition or structure of the tuber, has been frequently observed. Holt, in the Transactions of the Society Of Arts, mentions a variety called the " Dabb," large, coarse, and strong flavored, sad therefore unsuited lor the table, which became so much improved as to be no longer rejected.

Martin Doyle, in his Cyclopaedia of Agriculture, observes, that " the Irish Lumper is becoming every year more farinaceous and palatable." Other observations to the same effect, may be found in papers on the " Might" of the potato, published by the Highland Society Of Scotland. A remarkable change in the character of a value-variety, came under my own notice. In my youth, a large red, kidney shaped potato, known as the Scotch Red, or Flat Red, was most extensively cultivated, and almost universally esteemed, in consequence of its being very white and farinaceous when cooked. Having been from home a few years, I found on my return, a potato in use of very inferior quality - soft, watery, and of a yellow color, and was surprised to learn that this was the Scotch Red, which was formerly so excellent. On going through the grounds of a market gardener, soon after, a patch of potatoes with peculiar spindling stems, and scanty foliage, attracted my attention, and on inquiring the name of the variety, I learned that it was the Scotch Red; it had been so great a favorite, my friend remarked, that they were obliged to continue to grow a few, as some people would have them, but they could no longer rely upon it for a crop; many sets perished without vegetating, and it was now comparatively unproductive, and the potato worthless.

The quality or dry condition of the tubers of a given variety, may be influenced to a certain extent, by the nature of the soil and season; but the gradual alteration in the tubers of varieties, as above stated, is certainly of too general and progressive a character to be the exclusive result of any external influence; it is manifestly a consequence of the declining power of the inherent principle of life.

The change from a coarse, watery potato, fit only for cattle, to one so different as to be suitable for the food of man, is an event of too marked a character to pass unnoticed, even by the most careless; hence this change has been more particularly noticed in such varieties. But if coarse varieties of the potato are subject to this progressive change, is it not probable that all are governed by the same law; that the finer varieties must be similarly affected; may not those which from the first were comparatively dry and farinaceous, become in the course of time, and when growing under ordinary circumstances, still drier, their fluids thicker, and less abundant. If this be so, then it is no longer a mystery why " time or old age never fails to bring on the curled or shrivelled disorder;" why a variety in the earlier years of its existence may be healthy, and then become liable to the curl. We may also understand why some varieties of the potato may be over-ripened in England, and yet the plant be cultivated in the tropics; why cold, wet, and cloudy seasons, adverse to the growth of the potato, may produce tubers which afford the most healthy plants; and why a comparatively young variety may be subject to the curl, while another variety may become infirm and useless without exhibiting any marked symptoms of the disease.

The utility of preventing the perfect ripening of the seed-tubers of dry and farinaceous varieties, is apparent. Holt observed that " the finer kinds sooner degenerate than the coarse kinds, which are almost, if not always, the most productive, and retain their vigor the longest." The cause of this, too, must be now obvious. The dry and farinaceous tuber, as Mr. Knight observed, " indicates some degree of approximation to disease;" an observation evidently well founded.

The changes induced in the character of the potato by age, seem calculated to throw fresh light on the gradual deterioration or wearing out of trees. Trees afford, on consideration, the same evidence as potatoes, of progressive changes, leading to functional derangement and debility. We see in a variety of fruit trees, the vigor of youth, the productiveness of maturity, and the decripitude of age. These stages in the progress of life, are distinctly marked. The action of external influences cannot account for them. We may take three stocks of equal vigor, and graft on one a scion from a healthy tree just sprung from seed; on another a scion from a tree in the middle of life, and on the third a scion from an aged, almost worn out variety. Notwithstanding the equality of the stocks, the trees which spring from them will exhibit unequal degrees of vigor. One will grow with great luxuriance, and for some years show no disposition to bear fruit; the second will grow moderately, and soon bear fruit abundantly; whilst the third will shortly manifest all the symptoms of a decrepid old tree.