NATURAL SELECTION 139 



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 sev- 

 eral 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, 

 nevertheless 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 ren- 

 dered so highly attractive to insects that pollen was regu- 

 larly 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 an- 

 other 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 impo- 

 tent; now if we suppose this to occur in ever so slight 

 1 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 advanta- 

 geous 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 



