CHAP. XIV.] ITS NATURE AND ORIGIN. 231 



of a beech-tree, which it gracefully avoided; it was seen for a con- 

 siderable time sweeping round the beeches, and performing mag- 

 nificent evolutions in the air high above them. The other, which 

 was observed to beat the air with its wings more than usual, was 

 soon lost to sight behind some trees." He adds, " Titmice, tomtits, 

 and wrens I have made the subjects of a similar study, and with 

 similar results." 



The Plasticity of Instinct. 



The same careful observer says :* " Though the instincts 

 of animals appear and disappear in such seasonable corre- 

 spondence with their own wants and the wants of their 

 offspring as to be a standing subject of wonder, they have 

 by no means the fixed and unalterable character by which 

 some would distinguish them from the higher faculties of 

 the human race. They vary in the individuals as does 

 their physical structure. Animals can learn what they 

 did not know by instinct, and forget the instinctive know- 

 ledge which they never learned, while their instincts will 

 often accommodate themselves to considerable changes in 

 the order of external events.'* He then records the fol- 

 lowing experiment : 



" Everybody knows it to be a common practice to hatch ducks' 

 eggs under the common hen, though in such cases the hen has to 

 bit a week longer than on her own eggs. I tried an experiment to 

 ascertain how far the time of sitting could be interfered with in the 

 opposite direction. Two hens became broody on the same day, and 

 I set them on dummies. On the third day I put two chicks a day 

 old to one of the two hens. She pecked at them once or twice ; 

 seemed rather iidgety, then took to them, called them to her, and 

 entered on all the cares of a mother. The other hen was similarly 

 tried, but with a very different result. She pecked at the chickens 

 viciously, and both that day and the next stubbornly refused to 

 have anything to do with them." 



* " Nature," October 7, 1875, p. 507. 



