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 



