22 INTRODUCTION. 



reason ; that birds do change and improve when affected by 

 the same causes that make men do so ; and that mankind 1 

 neither alter nor improve when they exist under conditions, 

 similar to those which are almost universal among birds." 



Quite irreconcilable with the instinct theory is the fact 

 alluded to, that so many birds instead of building their 

 nests, according to instinct, take possession of ready-made 

 nests or holes, and make themselves at home therein. Thus 

 the puffin, Fratercula arctica, uses rabbit holes, after it has 

 turned out the rightful owners, and only decides to erect an 

 underground building on its own account if the other does 

 not suit it. The swift, Cypsehts apus, on its return in 

 spring time from warmer climes, ejects from its wonted 

 resorts sparrows, starlings and redstarts, and settles itself 

 down at home, using for its own purposes the stuff gathered 

 together for nests. The eggs and the unfledged young of 

 the legitimate owners are thrown out without mercy. That 

 the nest-building of birds does not rest on mere instinct is 

 further shown by the fact that, as already mentioned, old 

 birds build better nests than do young ones, and that young 

 birds brought up alone in cages either build very bad ones. 

 or no nests at all. 



