^"■gg^"] Coi-Es, GatJi.c\< Heligoland. 34^ 



cases alter their specific gravit}- to an extent which Giltke 

 witnesses without pretending to explain ; but it would seem that 

 they can do the same in the air, and that some of their modes of 

 flight, notably their sailing, are absolutely in defiance of much 

 that has been alleged and believed. Instead of answering the 

 question, "What guides birds during their migrations?" Giitke 

 says (p. 142) : — 



" Having thus examined the various attempts made to explain the 

 wonderful faculty possessed by migrants of discovering the right 

 path of their migrations, and shown how insufficient most of them 

 are when confronted with actual facts, observed directly in nature, 

 in the course of more than fifty years' investigations and at a 

 spot so favored as Heligoland, I cannot say that I feel encouraged 

 to add further to the number of such attempts by others of my 

 own." 



And for another modest disclaimer, which can be commended 

 to any who show symptoms of omniscience in the pleasing paths 

 of ornithology (p. 148) : — 



" Both in regard to this question as to the immediate departure 

 of birds on their migrations, as well as in reference to that pro- 

 pounded in the previous chapter [just cited], we are confronted 

 with a riddle which has hitherto defied every attempt at a solution, 

 and which indeed we may hardly expect will ever be likely to 

 receive a final explanation. Long and profound study has been 

 devoted to this subject in many quarters, and has resulted in the 

 enunciation of several very ingenious and plausible hypotheses. 

 None of these, however, will stand their ground wh>;n the actual 

 facts, which the life of birds in nature presents iu such abundance, 

 are marshalled against them. ... In treating of the various 

 momenta of the migratory flight, we have striven as much as 

 possible to place before the reader only such facts as are beyond 

 question or criticism, but no attempt has been made to furnish a 

 solution of the wide problems which they involve. In thus 

 abstaining from setting forth new theories, I have been guided 

 by the conviction, rendered firmer with increasing knowledge of 

 the phenomena, that what at present has been ascertained in 

 reference to the migration of birds furnishes us with no clue by 

 the aid of which we are enabled to penetrate the depths of this 

 wondrous mystery." 



44 



