Chap. IV. NATURAL SELECTION. 93 



When our plant, by this process of the continued 

 preservation or natural selection of more and more 

 attractive flowers, had been rendered highly attractive 

 to insects, they would, unintentionally on their part, 

 regularly carry pollen from flower to flower ; and that 

 they can most effectually do this, I could easily show 

 by many striking instances. I will give only one — not 

 as a very striking case, but as likewise illustrating one 

 step in the separation of the sexes of plants, presently 

 to be alluded to. Some holly-trees bear only male 

 flowers, which have four stamens producing rather a 

 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 pollen-grains, and on 

 some a profusion of pollen. 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 favour- 

 able to bees, nevertheless every female flower which 

 I examined had been effectually fertilised by the bees, 

 accidentally dusted with pollen, having flown from tree 

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

 imaginary case : as soon as the plant had been ren- 

 dered so highly attractive to insects that pollen was 

 regularly carried from flower to flower, another 

 process might commence. No naturalist doubts the 

 advantage of what has been called the "physiological 

 division of labour ; " hence we may believe that it would 

 be advantageous to a plant to produce stamens alone in 

 one flower or on one whole plant, and pistils alone in 



