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end, but unconsciously, or, in other words, by instinct. In 

 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. ^ The life of 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 enquiries from the 

 Standpoint which one has reached at its close.^' 



