CHAP. IV.] NATURAL SELECTION. 69 



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 dimorphism and other means, by which the separa- 

 tion 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 in- 

 termediate condition, or, as he expresses it, are more or less 

 dioaciously 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 ; and that 

 certain insects depended in main part on its nectar for food. I 

 could give many facts showing how anxious bees are to save 

 time : for instance, their habit of cutting holes and sucking the 

 nectar at the bases of certain flowers, which with a very little 

 more trouble, they can enter by the mouth. Bearing such facts 

 in mind, it may be believed that under certain circumstances 

 individual differences in the curvature or length of the proboscis, 

 tc., too slight to be appreciated by us, might profit a bee or other 

 insect, so that certain individuals would be able to obtain their 

 food more quickly than others ; and thus the communities to 

 which they belonged would flourish and throw off many swarms 

 inheriting the same peculiarities. The tubes of the corolla of the 

 common red and incarnate clovers (Trifolium pratense and incar- 

 natum) do not on a hasty glance appear to differ in length ; yet 

 the hive-bee can easily suck the nectar out of the incarnate 

 clover, but not out of the common red clover, which is visited by 

 humble-bees alone ; so that whole fields of the red clover offer ID 

 vain an abundant supply of precious nectar to the hive-bee. That 

 this nectar is much liked by the hive-bee is certain ; for I have 

 repeatedly seen, but only in the autumn, many hive-bees sucking 

 the flowers through holes bitten in the base of the tube by 

 humble-bees. The difference in the length of the corolla in the 



