346 THE STRUCTURE AND LIFE OF BIRDS -CcH. XII 
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” in Darwin’s Origin of Species. 
Romanes’ Mental Evolution in Animals and Animal [ntel- 
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. 
