148 



THE BIEDS OF HELIGOLAND 



Both in regard to this question as to the immediate cause of the 

 departure of birds on their migrations, as well as in reference to 

 that propounded in the previous chapter, 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 when the actual facts, which the life of birds in 

 Nature presents in such abundance, are marshalled against them. 



In one way or another, however, almost every attempt at an 

 explanation admits that migrants, with regard to the time and 

 direction of their movements, act with a means to an end, but 

 unconsciously, or, in other words, by instinct. 



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 know- 

 ledge 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. The life of a man is too short for the 

 complete exploration of this inexhaustible field, and one can only 

 regret that one is unable to start afresh with observations and 

 inquiries from the standpoint which one has reached at its close. 



