334 THE STRUCTURE AND LIFE OF BIRDS cHapP. 
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. He was 
at first inclined to believe that the young birds when 
still in the nest learnt the principles of architecture. 
This is as ifan 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. 
