But, first of all, the question needs to be more specifically stated.  There are varieties and varieties.  They may, some of them, disappear or deteriorate, but yet not wear out—­not come to an end from any inherent cause.  One might even say, the younger they are the less the chance of survival unless well cared for.  They may be smothered out by the adverse force of superior numbers; they are even more likely to be bred out of existence by unprevented cross-fertilization, or to disappear from mere change of fashion.  The question, however, is not so much about reversion to an ancestral state, or the falling off of a high-bred stock into an inferior condition.  Of such cases it is enough to say that, when a variety or strain, of animal or vegetable, is led up to unusual fecundity or of size or product of any organ, for our good, and not for the good of the plant or animal itself, it can be kept so only by high feeding and exceptional care; and that with high feeding and artificial appliances comes vastly increased liability to disease, which may practically annihilate the race.  But then the race, like the bursted boiler, could not be said to wear out, while if left to ordinary conditions, and allowed to degenerate back into a more natural if less useful state, its hold on life would evidently be increased rather than diminished.

As to natural varieties or races under normal conditions, sexually propagated, it could readily be shown that they are neither more nor less likely to disappear from any inherent cause than the species from which they originated.  Whether species wear out, i.e., have their rise, culmination, and decline, from any inherent cause, is wholly a geological and very speculative problem, upon which, indeed, only vague conjectures can be offered.  The matter actually under discussion concerns cultivated domesticated varieties only, and, as to plants, is covered by two questions.

First, Will races propagated by seed, being so fixed that they come true to seed, and purely bred (not crossed with any other sort), continue so indefinitely, or will they run out in time—­not die out, perhaps, but lose their distinguishing characters?  Upon this, all we are able to say is that we know no reason why they should wear out or deteriorate from any inherent cause.  The transient existence or the deterioration and disappearance of many such races are sufficiently accounted for otherwise; as in the case of extraordinarily exuberant varieties, such as mammoth fruits or roots, by increased liability to disease, already adverted to, or by the failure of the high feeding they demand.  A common cause, in ordinary cases, is cross-breeding, through the agency of wind or insects, which is difficult to guard against.  Or they go out of fashion and are superseded by others thought to be better, and so the old ones disappear.