ACTION OF NATURAL SELECTION 107 



plants. Some holly-trees bear only male flowers, which have 

 four stamens producing a rather small quantity 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 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 "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 separation 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 complete 

 separation of the sexes might be effected. It would take up 

 too much space to show the various steps, through dimorph- 

 ism 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 con- 

 dition, or, as he expresses it, are more or less dicEciously 

 polygamous. 



