Page - 226 - in The Origin of Species

226 ORIGIN OF SPECIES forms. In certain Umbelliferae the exterior seeds, according to Tausch, are orthospermous, and the central one coelosper- mous, and this is a character which was considered by De Candolle to be in other species of the highest systematic im- portance. Prof. Braun mentions a Fumariaceous genus in which the flowers in the lower part of the spike bear oval, ribbed, one-seeded nutlets ; and in the upper part of the spike, lanceolate, two-valved, and two-seeded siliques. In these several cases, with the exception of that of the well developed ray-florets, which are of service in making the flowers con- spicuous to insects, natural selection cannot, as far as we can judge, have come into play, or only in a quite subordinate manner. All these modifications follow from the relative position and inter-action of the parts ; and it can hardly be doubted that if all the flowers and leaves on the same plant had been subjected to the same external and internal con- dition, as are the flowers and leaves in certain positions, all would have been modified in the same manner. In numerous other cases we find modifications of structure, which are considered by botanists to be generally of a highly important nature, affecting only some of the flowers on the same plant, or occurring on distinct plants, which grow close together under the same conditions. As these variations seem of no special use to the plants, they cannot have been influenced by natural selection. Of their cause we are quite ignorant ; we cannot even attribute them, as in the last class of cases, to any proximate agency, such as relative position. I will give only a few instances. It is so common to observe on the same plant, flowers indifferently tetramerous, pentam- erous, &c., that I need not give examples ; but as numerical variations are comparatively rare when the parts are few, I may mention that, according to De Candolle, the flowers of Papaver bracteatum offer either two sepals with four petals (which is the common type with poppies), or three sepals with six petals. The manner in which the petals are folded in the bud is in most groups a very constant morphological character ; but Professor Asa Gray states that with some species of Mimulus, the aestivation is almost as frequently that of the Rhinanthidese as of the Antirrhinide?e, to which latter tribe the genus belongs. Aug. St. Hilaire gives the