Chap. VII.] THEOKY OF NATURAL SELECTION. 271 



or seeds of the circumference and centre sometimes 

 differ greatly in form, colour, and other characters. In 

 Carthamus and some other Composite the central 

 achenes alone are furnished with a pappus; and in 

 Hyoseris the same head yields achenes of three different 

 forms. In certain Umbelliferse the exterior seeds, 

 according to Tauseh, are orthospermous, and the central 

 one ccelospermous, and this is a character which was 

 considered by De Candolle to be in other species of the 

 highest systematic importance. Prof. Braun mentions 

 a Fumariaceous genus, in winch 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 conspicuous 

 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 rela- 

 tive 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 condition, 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 gener- 

 ally of a highly important nature, affecting only some 

 of the flowers on the same plant, or occurring on dis- 

 tinct 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 ; 



