Chap. VIL] THEORY OF NATURAL SELECTION. 271 



abortion of the reproductive organs. It is a more 

 curious fact, previously referred to, that the achenes 

 or seeds of the circumference and centre sometimes dif- 

 fer greatly in form, colour, and other characters. In Car- 

 thamus 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 

 Tausch, are orthospermous, and the central one coelo- 

 spermous, and this is a character which was considered 

 hy De CandoUe to be in other species of the highest 

 systematic importance. Prof. Braun mentions a Fu- 

 mariaceous 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 subor- 

 dinate 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 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 



