THEORY OP NATURAL SELECTION. 193 



the laws of growth, that I will give some additional cases 

 of another kind, namely of differences in the same part or 

 organ, due to differences in relative position on the same 

 plant. In the Spanish chestnut, and in certain fir-trees, 

 the angles of divergence of the leaves differ, according to 

 Schacht, in the nearly horizontal and in the upright 

 branches. In the common rue and some other plants, one 

 flower, usually the central or terminal one, opens first, and 

 has five sepals and petals, and five divisions to the ovarium ; 

 while all the other flowers on the plant are tetramerous. 

 In the British Adoxa the uppermost flower generally has 

 two calyx-lobes with the other organs tetramerous, while 

 the surrounding flowers generally have three calyx-lobes 

 with the other organs pentamerous. In many compositse 

 and umbelliferse (and in some other plants) the circum- 

 ferential flowers have their corollas much more developed 

 than those of the centre ; and this seems often connected 

 with the 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 differ greatly in 

 form, color, 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 Tausch, are orthospermous, 

 and the central one coelospermous, and this is a character 

 which was considered by De Candolle to be in other species 

 of the highest systematic importance. Professor 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 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 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 struc- 

 ture, which are considered by botanists to be generally of a 



