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Show 340 DARWINIAN A. natural if less useful state, its hold on life would evidently be increased r~ther 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 originat~d. 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 pr_opagated by seed, being so iixed that they come true to seed, and purely bred (not crossed with any other sort), continue so inde:finitely, or will they r·un mtt 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 accoilllted 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 Or they go out of fashion and are superseded by others thought to be better, and so the old ones disappear. Or, finally, they may revert to an ancestral form. DURATION OF RACES. 341 As offspring tend to resemble grandparents almost as ~nuch as parents, and as a line of clos(3-bred ancestry IS generally prepotent, so newly-originated varieties have always a tendency to reversion. This is pretty sure to show itilelf in some of the proo·eny of the eai:· ~ier ge~e~·ations, ~nd the breeder has ~o guard against It by .rigid selectwn. But the older ·the· variety isthat Is, the longer the series of generations in which it has come true from seed-the less the chance of re-· · version: for now, to be like the immediate parents is ~]so to be like a long line of ancestry; and so all ~he m:fiuences concerned-that is, both parental and ancestral heritability-act in one and the same direction. So, since the older a race is the more reason it has to continue true, the presumption of the unlimited permanence of old races is very strong. Of course the race itself may give off new varieties; but that is no interference with the vitality of the original stock. If some of the new vaTieties supplant the old, that will not be because the unvaried stock is wo~n out or decrepit with age, but because in wild Nature the newer forms are b~tter adapted to the surroundings, or, under man's care, better adapted to his wants or fancies. The second question, and one upon which the discussion about the wearing out of varieties generally turns, is, Will varieties propagated from bud8, i. e., by division, grafts, bulbs, tubers, and tlw like, nece8sarily deter?." orate and die mtt? First, Do they die out as a matter of fact? Upon this, the testimony has all along been conflicting. Andrew Knight was sure that they do, and there coul~ hardly be a more trustworthy witness. |
