THE GLACIAL RANGE CONTRACTION, ETC. 165 



its area of distribution southwards, either in summer 

 or winter ; if it be overtaken by a change to a colder 

 climate in the north, the individuals of the species 

 that occupy that area will assuredly perish, and that 

 portion of the species be eradicated.^ If, for instance, 

 the British Islands are visited by an exceptionally 

 severe winter, birds, instead of flying off south to escape 

 its terrors, die helplessly in thousands and tens of 

 thousands ; and it has been remarked that certain 

 localities have been well-nigh depopulated of various 

 birds, not perhaps regaining their usual strength again 

 for years. If the birds, mstead of perishing from cold 

 and hunger, had merely emigrated south a little way 

 (perhaps but a very few miles !) they would have been 

 safe, and the return of spring weather would have 

 brought them back in their wonted numbers again. 

 We have at least some direct evidence that the cold 

 winters we have been so persistently having of late 

 years are slowly exterminating the Dartford Warbler. 

 As a British species it is undoubtedly much rarer than 

 it was twenty years ago. It is sedentary, and if these 

 conditions continue nothing can save this little bird 

 from total extinction in the British Islands. We 

 observe no extension of range southwards, no " re- 



^ A species may become gradually extinct through the inability 

 to rear offspring owing to adverse climatic conditions. In some 

 seasons, for instance, the Brent Goose appears on our coasts 

 unaccompanied by any young ; and it may fairly be presumed 

 that an unfavourable season (say abnormally cold, backward, or 

 stormy) in the high latitudes where this species breeds has proved 

 fatal to the eggs or young. Should these adverse climatic con- 

 ditions become more pronounced, and continue season after season 

 (as they must have done during the coming on of a Glacial Period) 

 the species would most assuredly gradually be e.xterminated. 



