204 ANIMALS AT WORK AND PL A ? 



regions from which they are supposed once to have 

 been driven by the advancing ice, is not new either 

 in idea or form. But whether c impulse ' or ' instinct ' 

 sends them northward, Mr Dixon attempts no explana- 

 tion of the exceptional power which enables them 

 to achieve their journey. He even suggests that 

 they need none. 'Once a bird begins its migration/ 

 he writes, ' all instinct as a guiding medium ceases ; 

 memory and knowledge of locality, in fact, experience, 

 assist it to perform that long journey/ Until this 

 conclusion is supported by more evidence it may 

 safely be neglected. Whatever the c impulse ' which 

 sends them out, it cannot yet be shown that it is 

 not instinct that shows them the way. It is not 

 instruction, for no parent bird could leave with its 

 young in England a way-bill for the Nile Valley, 

 though the young birds may possibly find their 

 way here in the wake of the old ones. It is not 

 example, for though many species travel in flock, 

 with old and young together, such as the swallows, 

 others force their young to migrate separately, and 

 Temminch states that the flocks of old and young 

 always travel separately. Other birds force the 

 young to migrate while they remain on their breed- 

 ing ground, or, as in the case of the cuckoo, leave 

 England a r month before the young. The 'follow- 

 my-leader' instinct accounts for much of the marvel 



