116 NATURAL SELECTION v 



'Parsons Pond,' in Newfoundland, which is separated from 

 the sea only by a high pebbly beach. Within the period 

 above stated high tides and heavy seas have shifted the course 

 of the brook flowing from the lake into the sea, and caused a 

 greater, and consequently a more rapid fall of fresh water, 

 which has so shallowed that part of the lake where the gulls 

 were in the habit of breeding that it was no longer safe to 

 build on rocks easily accessible to their common enemy, the 

 fox. They therefore betook themselves to some neighbouring 

 spruce and balsam firs not much over a hundred yards distant 

 from their old breeding station." Audubon also notes a 

 similar change of habit, some herring-gulls building their nests 

 in spruce-trees on an island in the Bay of Fundy, where they 

 had formerly built on the ground. 



A curious example of a recent change of habits has oc- 

 curred in Jamaica. Previous to 1854 the palm swift 

 (Tachornis phoenicobea) inhabited exclusively the palm trees 

 in a few districts in the island. A colony then established 

 themselves in two cocoa-nut palms in Spanish Town, and 

 remained there till 1857, when one tree was blown down and 

 the other stripped of its foliage. Instead of now seeking out 

 other palm trees the swifts drove out the swallows who built 

 in the piazza of the House of Assembly, and took possession 

 of it, building their nests on the tops of the end walls and at 

 the angles formed by the beams and joists, a place which they 

 continue to occupy in considerable numbers. It is remarked 

 that here they form their nest with much less elaboration than 

 when built in the palms, probably from being less exposed. 



But perfection of structure and adaptation to purpose are 

 not universal characteristics of birds' nests, since there are 

 decided imperfections in the nesting of many birds which are 

 quite compatible with our present theory, but are hardly so 

 with that of instinct, which is supposed to be infallible. The 

 passenger pigeon of America often crowds the branches with 

 its nests till they break, and the ground is strewn with 

 shattered nests, eggs, and young birds. Books' nests are 

 often so imperfect that during high winds the eggs fall out ; 

 but the window-swallow is the most unfortunate in this re- 

 spect, for White, of Selborne, informs us that he has seen them 

 build, year after year, in places where their nests are liable 



