96 ILLUSTRATIONS OF THE ACTION OF Chap. IV. 



which produced more and more pollen, and had larger anthers, 

 would be selected. 



"When our plant, by the above process long continued, had 

 been rendered highly attractive to insects, they would, unin- 

 tentionally on their part, regularly carry pollen from flower to 

 flower; and that they do this cfl'ectually, I could easily show 

 l)y many striking facts. I will give only one, as likewise illus- 

 trating one step in the separation of the sexes of plants. Some 

 liolly-trees bear. only male flowers, which have four stamens 

 ])roducing a rather small quantity of pollen, and a rudimentary 

 l)istil ; 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 difierent 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 advanta- 

 geous 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, 

 tlien, 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 di\'ision of 

 labor, individuals with this tendency more and more increased, 

 would be continually favored or selected, until at last a eoin- 

 phite separation of the sexes might be effected. It would 

 take up too much space to show the various steps, through 

 diinorpliism and other means, by which the separation of the 

 sexes in plants of various kinds is aiiparently now in progress ; 



