THEORY OF NATURAL SELECTION 225 



feet flowers), the sepals are reduced from the normal num- 

 ber of five to three. In one section of the Malpighiacca; the 

 closed flowers, according to A. de Jussieu, are still further 

 modified, for the five stamens which stand opposite to the 

 sepals are all aborted, a sixth stamen standing opposite to 

 a petal being alone developed ; and this stamen is not present 

 in the ordinary flowers of these species; the style is aborted; 

 and the ovaria are reduced from three to two. Now although 

 natural selection may well have had the power to prevent 

 some of the flowers from expanding, and to reduce the amount 

 of pollen, when rendered by the closure of the flowers super- 

 fluous, yet hardly any of the above special modifications can 

 have been thus determined, but must have followed from 

 the laws of growth, including the functional inactivity of 

 parts, during the progress of the reduction of the pollen and 

 the closure of the flowers. 



It is so necessary to appreciate the important effects of 

 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 ; whilst 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, whilst the surrounding flowers 

 generally have three calyx-lobes with the other organs pen- 

 tamerous. In many Composite and Umbelliferae (and in 

 some other plants) the circumferential flowers have their 

 corollas much more developed than those of the centre; 

 and this seems often connected with the abortion of the re- 

 productive 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, colour, and 

 other characters. In Carthamus and some other Compositre 

 the central achenes alone are furnished with a pappus ; and 

 in Hyoseris the same head yields achenes of three different 



II— [[(• XI 



