OF NATURAL SBLBGTION. 87 



sexes of plants. Some holly-trees bear only male flowers, 

 which have four stamens producing a rather small quan- 

 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 polen 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 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, 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 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 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, though 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 diceciously 

 polygamous. 



