NATURAL SELECTION 85 



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 

 rendered 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 another flower or on another plant. In plants 

 under culture and placed under new conditions of life, 

 sometimes the male organs and sometimes the female 

 organs become more or less impotent ; now if we 

 suppose this to occur in ever so slight a degree under 

 nature, then as pollen is already carried regularly 

 from flower to flower, and as a more complete separa- 

 tion of the sexes of our plant would be advantageous 

 on the principle of the division of labour, individuals 

 with this tendency more and more increased, would be 

 continually favoured or selected, until at last a com- 

 plete separation of the sexes would be effected. 



Let us now turn to the nectar-feeding insects in our 

 imaginary case : we may suppose the plant of which 

 we have been slowly increasing the nectar by continued 

 selection, to be a common plant ; and that certain 

 insects depended in main part on its nectar for food. 

 I could give many facts, showing how anxious bees are 

 to save time ; for instance, their habit of cutting holes 

 and sucking the nectar at the bases of certain flowers, 

 which they can, with a very little more trouble, enter 



