164 



THE BIRDS OF HELIGOLAND 



myself to treating only of the small number of examples in regard to 

 which I can speak with positive assurance, confining my description 

 to what actually takes place, without embarking on any hypotheti- 

 cal conjectures as to the how and wherefore of the matter; for if it 

 indeed should ever be sought to trace the origin and course of the 

 process, this end could only be attained by way of the most 

 searching microscopic examination. 



