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The giant evening-primrose, though not taller in stature than O. lamarckiana, deserves its name because it is so much stouter in all respects. The stems are robust, often with twice the diameter of lamarckiana throughout. The internodes are shorter, and the leaves more numerous, covering the stems with a denser foliage.
Further promise of yellow beauty was given by the stalks of the evening-primrose scattered on every hand, the flowers furled now, sleeping. In the groves were pines, small cedars, and a sprinkling of sturdy dwarf oaks. And from their shelter came the welcome sound of a bird's twitter. "It's always about as you see it," Norton explained.
I have already given the names of the only two forms, which deserve to be mentioned here. One of them is called scintillans or the shiny evening-primrose, because its leaves are of a deep green color with smooth surfaces, glistening in the sunshine. On the young rosettes these leaves are somewhat broader, and afterwards somewhat narrower than those of O. lamarckiana at the corresponding ages.
It was not really a native plant, but one that had been introduced from America and belongs to an American genus. I refer to the great evening-primrose or the evening-primrose of Lamarck. A strain of this beautiful species is growing in an abandoned field in the vicinity of Hilversum, at a short distance from Amsterdam. Here it has escaped from a park and multiplied.
Instead of the vague notions, uncertain hopes, and a priori conceptions, that have hitherto confused the investigator, methods of observation have been formulated, suitable for the attainment of definite results, the general nature of which is already known. To my mind the real value of the discovery of the mutability of the evening-primrose lies in its usefulness as a guide for further work.
Sometimes they fertilize themselves without any aid, as for instance, the common evening-primrose; in other cases the pollen has to be placed on the stigma artificially, as with Lamarck's evening-primrose and its derivatives. Other plants need cross-fertilization in order to produce a normal yield of seeds.
On the other hand, the hypothesis of mutative periods is by no means irreconcilable with the observed facts of constancy. Such casual changes can be proved by observations such as those upon the evening-primrose, but it is obvious that a disproof can never be given.
All hypotheses concerning the direct causes of adaptation at once become superfluous, and the great principle enunciated by Darwin once more reigns supreme. In this way too, the mutation-period of the evening-primrose is to be considered as a prototype. Assuming it as such provisionally, it may aid us in arranging the facts of descent so as to allow of a deeper insight and a closer scrutiny.
In the case of the evening-primrose the systematic arrangement of the allied species readily guides us in the delimitations of such periods. For manifestly the species of the large genus of Oenothera are grouped in swarms, the youngest or most recent of which we have under observation.
I might give one more instance from my own experience. A variety of the evening-primrose with small linear petals was once found by one of my sons growing wild near Amsterdam. It was represented by only one individual, flowering among a great many of the ordinary type with broad petals.
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