74 THE ORIGIN OF SPECIES 



tity 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 de- 

 tected. 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 favorable to bees, neverthe- 

 less every female flower which I examined had been effectually 

 fertilized 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 "physiological division of labor;" 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 some- 

 times 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 separation of the sexes of our plant would be 

 advantageous on the principle of the division of labor, individuals 

 with this tendency more and more increased would be continually 

 favored 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 inter- 

 mediate condition, or, as he expresses it, are more 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 cer- 

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

 by the mouth. Bearing such facts in mind, it may be believed that 



