SUMMARY 143 



of years ago. We cannot conceive of the essential 

 requirements of birds of today being different in 

 any fundamental respect from those of their an- 

 cestors of byegone ages. 



The experiments in migration, of which some 

 account has been given in the pages above, are the 

 outcome of a critical examination, in light of modern 

 biological knowledge, of the available field facts 

 and existing theories of migration. The arguments 

 on which they are based apparently fit the facts, 

 but it must be borne in mind that any theory that 

 is new must remain subject to correction until 

 fully substantiated by repeated effort and the 

 critical investigations of fellow workers looking at 

 the problem from some other angle. The attitude 

 of the scientist must be entirely impartial and 

 without bias. He sets out to test a given hy- 

 pothesis, not to prove it or to disprove it. It is 

 not his business to make the pendulum swing in 

 any particular direction, but to watch and measure 

 its swing and to record its observed amplitude. His 

 mind must ever be open. 



Experimental biology, however, is peculiar in 

 this respect, that the investigator must of necessity 

 have a preconceived theory upon which to work. 

 He must have something concrete in his mind in 

 order to organise his line of attack. But it should 



