INTRODUCTORY 11 



the evidence against it (if at present somewhat 

 meagre) is quite sufficiently conclusive and un- 

 answerable. Like many other popular beliefs, it is 

 founded upon tradition and myth. The theory of an 

 unchanging instinct or unvarying inherited habit is 

 disproved by the fact that birds do very frequently 

 choose a site for their nest which differs in many 

 respects from the one usually selected by the species, 

 instances of which must be familiar to every observer; 

 and by persistence in it, if found to be advantageous 

 or in no way injurious an entirely new nesting habit 

 may result. Then again, although the fact is perhaps 

 not so generally known, many birds have not only 

 changed their habits of nesting, but in some cases 

 have completely altered the type of their nest. Such 

 a change is entirely at variance with any inherited 

 habit, and shows that birds are constantly exercising 

 their mental powers in adapting themselves to 

 changing conditions of life. Even in the British 

 Islands no less than five species can be named 

 which are known to have changed their mode of 

 nest-building considerably during a comparatively 

 short lapse of time. The House Martin there can be 

 little doubt before the dawn of civilisation in this area 

 attached its nest to maritime and inland cliffs. But 

 with the prevalence of a more elaborate form of 

 human architecture, the Martin, with an ever alert 

 sense of adaptiveness, acquired the habit of attaching 

 its nest to the most suitable portions of the new and 



