MIGRATION 171 



(a) It has been suggested that birds take advantage of 

 all sorts of landmarks — mountain-ranges, river- valleys, 

 coast-lines, chains of island. As birds have very acute 

 vision, it would be hasty to rule out this suggestion, but it 

 cannot be more than part of the solution. For many birds 

 migrate during the night and many cross great tracts of 

 sea which afford no landmarks. It is possible that there 

 may be some sensory acuteness apart from vision, but the 

 experiments that have been made have failed to detect this, 

 and have made it improbable that the nostrils or ear-openings 

 are of any importance in this connection. 



(b) It has seemed to some naturalists that the success 

 attending migration must be due to the cumulative inheri- 

 tance of the results of experience. This is the attractive 

 Lamarckian view, but it meets several serious difficulties. 

 It is uncertain that the results of individual experience 

 can be entailed on the offspring, and it is not evident what 

 the content of experience could be in birds flying by night, 

 at great heights, and over the trackless sea. In regard to 

 the second of these difficulties it is open to the Lamarckian 

 to say that the flying by night and at great heights and 

 over the trackless sea is the final outcome of ages of evolution, 

 and that one must think back to much humbler beginnings 

 when the migrants flew by day and low and from island to 

 island. Then, obviously, the experience would have a 

 definite content. There remains the difficulty of the 

 transition from the humble beginnings to the astounding 

 outcome, the difficulty of finding evidence to warrant a 

 belief in the cumulative results of individually acquired 

 experience. 



(c) A third suggestion lays emphasis on the social 

 character of many migrations and suggests that a tradition 

 may be kept up. Thus those who followed well for several 

 years may become the leaders in the course of time. It 

 would be unwarranted to say that tradition does not 

 count in migration, but it cannot be more than a subsidiary 

 factor. For it sheds little light on the original attain- 

 ment of efficiency and it does not fit in well with the 



