Chap. IY.] NATUKAL SELECTION. 115 



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 their part, regularly carry 

 pollen from flower to flower; and that they do this 

 effectually, I could easily show by many striking facts. 

 I will give only one, as likewise illustrating one step in 

 the separation of the sexes of plants. Some holly-trees 

 bear only male flowers, which have four stamens produ- 

 cing a rather small quantity of pollen, and a rudimentary 

 pistil ; other holly -trees bear only female flowers ; these 

 have a full-sized pistil, and four stamens with shrivelled 

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

 Having found a female tree exactly sixty yards from a 

 male tree, I put the stigmas of twenty flowers, taken from 

 different branches, under the microscope, and on all, 

 without exception, there were a few pollen-grains, and on 

 some a profusion. As the wind had set for several days 

 from the female to the male tree, the pollen could not 

 thus have been carried. The weather had been cold and 

 boisterous, and therefore not favourable to bees, never- 

 theless every female flower which I examined had been 

 effectually fertilised by the bees, which had flown from 

 tree to tree in search of nectar. But to return to our 

 imaginary case : as soon as the plant had been rendered 

 so highly attractive to insects that pollen was regularly 

 carried from flower to flower, another process might com- 

 mence. No naturalist doubts the advantage of what has 



