334 THE STRUCTURE AND LIFE OF BIRDS chap. 



Nest-building. 



Nest-building is generally held to be entirely the 

 work of instinct. But Dr. Wallace has tried to show 

 that this too is an acquired accomplishment. 1 He was 

 at first inclined to believe that the young birds when 

 still in the nest learnt the principles of architecture. 

 This is as if an infant in arms on seeing a steam-engine 

 should at once understand how it is made. Giving up 

 this theory he suggests other possibilities — that, when 

 they first have to build they see another pair at work 

 and so learn, or that a young bird always pairs with 

 an old one. These views will hardly bear examination. 

 If we wish to get at the true explanation, we must 

 realise that instinct is plastic and can be modified by 

 reason. Birds frequently, as Dr. Wallace says, show, 

 when they are building their nests, that they are not 

 mere machines. They adapt themselves to new 

 situations. The Swallow and the House-Martin have 

 availed themselves of barns and houses. The Palm 

 Swift in Jamaica till 1854 always built in palms. But 

 in Spanish Town when two cocoanut palms were 

 blown down, they drove out the Swallows from the 

 Piazza of the House of Assembly and built between 

 the angles formed by the beams and joists. In 

 America the Tailor-bird now uses thread and worsted 

 for its nest instead of wool and horsehair, and wool and 

 horsehair may originally have been substitutes for vege- 

 table fibres and grasses. In Calcutta an unconven- 



1 See Dr. Wallace's Contributions to the Theory of Natural 

 Selection^ p. 211. 



