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Responses were supposed to be produced, but the corresponding outward changes would be too small to betray themselves to the investigator. The direct observation of the mutations of the evening-primrose has changed the whole aspect of the problem at once. It is no longer a matter dealing with purely hypothetical conditions.
New elementary species are far more rare, but I have discovered in the great evening-primrose, or Oenothera lamarckiana a strain which is producing them yearly in the wild state as well as in my garden. These observations and pedigree-experiments will be dealt with at due length in subsequent lectures.
Lamarck's evening-primrose is a stately plant, with a stout stem, attaining often a height of 1.6 meters and more. When not crowded the main stem is surrounded by a large circle of smaller branches, growing upwards from its base so as often to form a dense bush. These branches in their turn have numerous lateral branches.
The large-flowered evening-primrose was also cultivated about the beginning of the last century in the gardens of the Museum d'Histoire Naturelle, at Paris, where it was noticed by Lamarck, who at once distinguished it as an undescribed species.
One is the mutability of Lamarck's primrose, and the second is the immutable condition of quite a number of other species. Among them are some of its near allies, the common and the small flowered evening-primrose, or Oenothera biennis and O. muricata. From these facts, a very important question arises in connection with the theory of descent.
I immediately went down to the edge of a bushy and swampy meadow below our camp and brought him a four-petaled flower of galium, and a plant-stalk with four leaves in a whorl. In another locality I might have brought him dwarf cornel, or the houstonia, or wood-sorrel, or the evening-primrose.
Considering the mutative period of our evening-primrose as one unit-stride section in the great genealogic tree, this period includes two nearly related, but not identical changes. One is the production of new specific characters in the latent condition, and the other is the bringing of them to light and putting them into active existence.
In my garden it has proved to be constant from seed, never reverting to the original lamarckiana, provided intercrossing was excluded. It is chiefly distinguished from Lamarck's evening-primrose by its smooth leaves, as the name indicates. The leaves of the original form show numerous sinuosities in their blades, not at the edge, but anywhere between the veins.
Should this be granted for the evening-primrose, it would have to be predicated for other species found in a mutable state. Then, of course, it would be useless to investigate the causes of mutability at large, and we should have to limit ourselves to the testing of large numbers of plants in order to ascertain which are mutable and which not.
Two cases of sudden mutations have come to my knowledge, producing this same anomaly in allied species. One has been already alluded to; it pertains to the common evening-primrose or Oenothera biennis, and one is a species belonging to another genus of the same family, the great hairy willow-herb or Epilobium hirsutum.
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