Nest-building, Incubation, and Migration. 261 



to migration are presumably in part external and in 

 part internal. "When we say that the external factor is 

 probably due to changes of climate and food supply, while 

 the internal factor is probably largely due to changes in 

 the reproductive system, we have said well nigh all that is 

 to be said — and that is little enough. In any case, the birds 

 are marvellously true to time, especially such sea birds as 

 the puffin, from which we may perhaps conclude that the 

 internal factor is the main determinant, since both climatic 

 changes and food supply are variable from year to year. 



With regard to the question of direction, the sense 

 whose organ is the semicircular canals, which are well 

 developed in birds, has been invoked as a source of 

 guidance. But it must be remembered that this organ, 

 like others of the special senses, while it affords data to 

 experience, finds in that experience its raison d'etre. By 

 themselves, as their action is at present understood, they 

 could afford no foundation for the directed flight of the 

 swallow from England to Natal, if this be truly instinctive. 

 The semicircular canals might enable the bird to continue 

 over wide seas a line once adopted ; but unless there is in 

 addition some sense of orientation (some equivalent in the 

 bird's organization to the mariner's compass) it is difficult 

 to see how they could afford instinctive guidance. 



The outcome of any consideration of the whole question 

 is a profound sense of ignorance. So great is that ignor- 

 ance, that it is difficult even to speculate. Were I pressed 

 to hazard a guess, I should be inclined to surmise that, 

 notwithstanding the observations of Herr Gatke, while the 

 migratory impulse is innate, and perhaps there is an 

 instinctive tendency to start in a given direction, yet the 

 element of traditional guidance may be effectual, in the 

 migration stream as a whole, in some way that we have 

 hitherto been unable to observe. 



