74 ILLUSTRATIONS OF THE ACTION OF [Chap. IV. 



Having found a female tree exactly sixty yards from a male tree. 1 

 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, nevertheless 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 commence. No 

 naturalist doubts the advantage of what has been called the " physio- 

 logical 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 m 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 sepa- 

 ration of the sexes of our plant would be advantageous on the prin- 

 ciple of the division of labour, individuals with this tendency more 

 and more increased, would be continually favoured or selected, 

 until at last a complete separation of the sexes might be effected. 

 It would take up too much space to show the various steps, 

 through dimorphism and other means, by which the separation of 

 the sexes in plants of various kinds is apparently now in progress ; 

 but I may add that some of the species of holly in North America, 

 are, according to Asa Gray, in an exactly intermediate condition, or, 

 as he expresses it, are more or less dioeciously polygamous. 



Let us now turn to the nectar-feeding insects ; 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 with a very little more trouble, 

 they can enter by the mouth. Bearing such facts in mind, it may 

 be believed that under certain circumstances individual differences- 

 in the curvature or length of the proboscis, &c, too slight to be 

 appreciated by us, might profit a bee or other insect, so that, 

 certain individuals would be ablo to obtain their food more quickly 



