346 THE STRUCTURE AND LIFE OF BIRDS CH. XIII 



occasionally a single-flowered plant. The seed from 

 this produces plants, most of which bear double, but 

 a few of them single flowers. The barren double 

 flowers correspond to the neuter bees or workers, the 

 single ones to the queen. 



Some of the Literature of the Subject. 



The chapter on " Instinct 1 ' in Darwin's Origin of Species. 



Romanes' Mental Evolution in Animals and Animal Intel- 

 ligence. 



Wallace's Contributions to the Theory of Natural Selection. 



Sclater and Hudson's Argentine Ornithology (on " Parasitic 

 Birds.") 



Various books and papers to which references have been given 

 in the course of this chapter ; see footnotes. 



