Chap. IV 



Natural Selection. 



73 



district, competing with and conquering the unchanged individuals 

 on the margins of an ever-increasing circle. 



It may he worth while to give another and more complex illus- 

 tration of the action of natural selection. Certain plants excreta- 

 sweet juice, apparently for the sake of eliminating something in- 

 jurious from the sap : this is effected, for instance, by glands at the 

 base of the stipules in some Leguminosa3, and at the backs of the 

 leaves of the common laurel. This juice, though small in quantity, 

 is greedily sought by insects ; but their visits do not in any way 

 benefit the plant. Now, let us suppose that the juice or nectar was 

 excreted from the inside of the flowers of a certain number of plant s- 

 of any species. Insects in seeking the nectar would get dusted with: 

 pollen, and would often transport it from one flower to another. 

 The flowers of two distinct individuals of the same species would 

 thus get crossed ; and the act of crossing, as can be fully proved,, 

 gives rise to vigorous seedlings, which consequently would have the 

 best chance of flourishing and surviving. The plants which produced 

 flowers with the largest glands or nectaries, excreting most nectar,, 

 would oftenest be visited by insects, and would oftenest be crossed ; 

 and so in the long-run would gain the upper hand and form a local 

 variety. The flowers, also, which had their stamens and pistils- 

 placed, in relation to the size and habits of the particular insect 

 which visited them, so as to favour in any degree the transportal of 

 the polltn, would likewise be favoured. We might have taken the 

 case of insects visiting flowers for the sake of collecting pollen in- 

 stead of nectar; and as pollen is formed for the sole purpose of 

 fertilisation, its destruction appears to be a simple loss to the plant ; 

 yet if a little pollen were carried, at first occasionally and then 

 habitually, by the pollen-devouring insects from flower to flower, 

 and a cross thus effected, although nine-tenths of the pollen were 

 destroyed, it might still be a great gain to the plant to be thus 

 robbed ; and the individuals which produced more and more pollen, 

 and had larger anthers, would be selected. 



When our plant, by the above process long continued, had been 

 rendered highly attractive to insects, they would, unintentionally on 

 tbeir part, regularly carry pollen from flower to flower ; and that 

 they do this effectually, I could easily show by many striking facts. 

 1 will give only one, as likewise illustrating one step in the separa- 

 tion of the sexes of plants. Some holly-trees bear only male flowers,.. 

 which have four stamens producing a rather small quantity of 

 pollen, and a rudimentary pistil; other holly-trees bear only female- 

 nowers; these have a full-sized pistil, and four stamens with, 

 snivelled anthers, in which not a grain of pollen can be detected 



