Dioecious plants, however fertilised, have a great advantage over other plants in their cross-fertilisation being assured.  But this advantage is gained in the case of anemophilous species at the expense of the production of an enormous superfluity of pollen, with some risk to them and to entomophilous species of their fertilisation occasionally failing.  Half the individuals, moreover, namely, the males, produce no seed, and this might possibly be a disadvantage.  Delpino remarks that dioecious plants cannot spread so easily as monoecious and hermaphrodite species, for a single individual which happened to reach some new site could not propagate its kind; but it may be doubted whether this is a serious evil.  Monoecious plants can hardly fail to be to a large extent dioecious in function, owing to the lightness of their pollen and to the wind blowing laterally, with the great additional advantage of occasionally or often producing some self-fertilised seeds.  When they are also dichogamous, they are necessarily dioecious in function.  Lastly, hermaphrodite plants can generally produce at least some self-fertilised seeds, and they are at the same time capable, through the various means specified in this chapter, of cross-fertilisation.  When their structure absolutely prevents self-fertilisation, they are in the same relative position to one another as monoecious and dioecious plants, with what may be an advantage, namely, that every flower is capable of yielding seeds.

CHAPTER XI.

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS.

Insects visit the flowers of the same species as long as they can. 
Cause of this habit. 
Means by which bees recognise the flowers of the same species. 
Sudden secretion of nectar. 
Nectar of certain flowers unattractive to certain insects. 
Industry of bees, and the number of flowers visited within a short time. 
Perforation of the corolla by bees. 
Skill shown in the operation. 
Hive-bees profit by the holes made by humble-bees. 
Effects of habit. 
The motive for perforating flowers to save time. 
Flowers growing in crowded masses chiefly perforated.

Bees and various other insects must be directed by instinct to search flowers for nectar and pollen, as they act in this manner without instruction as soon as they emerge from the pupa state.  Their instincts, however, are not of a specialised nature, for they visit many exotic flowers as readily as the endemic kinds, and they often search for nectar in flowers which do not secrete any; and they may be seen attempting to suck it out of nectaries of such length that it cannot be reached by them. (11/1.  See, on this subject Hermann Muller ‘Befruchtung’ etc. page 427; and Sir J. Lubbock’s ‘British Wild Flowers’ etc. page 20.  Muller ‘Bienen Zeitung’ June 1876 page 119, assigns good reasons for his belief that bees and many other