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At all events the term adaptation includes the idea of utility, and obviously useless contrivances could hardly be brought under the same head. We have also dealt with the question of heredity. It is obvious that from the flowers of the floating and erect stems of the water-persicaria seeds will result, each capable of yielding both forms. Quite the same thing was the case with the teasels.
Manifestly this classification is as nearly right as that of the two varieties of the water-persicaria. Going one step further, we meet with the very interesting case of alpine plants. The vegetation of the higher regions of mountains is commonly called alpine, and the plants show a large number of common features, differentiating them from the flora of lower stations.
Among all the previously described cases of horticultural plants and monstrosities there is no clearer case of an ever-sporting variety than this one of the water-persicaria. The var. terrestris sports into the var. natans, and as often as the changing life conditions may require it. It is-true that ordinary sports occur without our discerning the cause and without any relation to adaptation.
It is readily granted that many writers would not willingly accept this conclusion. But it is simply impossible to avoid it. The two forms of the water-persicaria must remain varieties, though they are only types of the different branches of a single plant.
Exactly the same thing is true of double adaptations. Every bud of the water-persicaria may develop either into an erect or into a floating stem, according as it is surrounded by water or by relatively dry soil. In other cases utility is often less manifest, but some use may either be proved, or shown to be very probable.
The external conditions decide which of them becomes active and which remains inactive, and the case seems to be exactly parallel to that of the water-persicaria. In the experiments of Bonnier the influence of the soil was, as a rule, excluded by transplanting part of the original earth with the transplanted half of the plant.
Once made, this choice is final, and a further change does not occur in the normal course of things. The most curious and most suggestive instance of such an alternation is the case of the water-persicaria or Polygonum amphibium. It is known to occur in two forms, one aquatic and the other terrestrial.
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