82 ILLUSTRATIONS OF THE ACTION 



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

 have four stamens producing a rather small quantity of pol- 

 len, 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 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 micro- 

 scope, 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 onr 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 nat- 

 uralist 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 cul- 

 ture 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 com- 

 plete separation of the sexes of our plant would be advan- 

 tageous on the principle of the division of labor, individuals 

 with this tendency more and more increased would be con- 

 tinually favored or selected, until at last a complete separa- 

 tion 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 Amer- 

 ica are, according to Asa Gray, in an exactly intermediate 

 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 ; 



