VARIATION 297 



Purple Sandpiper — 



In the matter of coloration most birds show a striking 

 constancy, variations, which are always present, being very- 

 slight — whilst some species, on the contrary, present a very 

 remarkable range of variability ; a few display the phenomenon 

 known as discontinuous variation. 



In the matter of continuous variation the Common Partridge 

 {Perdix cinerea) and the Red Grouse afford exceptionally good 

 subjects for study, exhibiting a truly surprising scale of grada- 

 tions in coloration. More striking, perhaps, is the variation 

 exhibited by the Arctic Skua {Stercorarius crepidatus), which 

 displays two very distinct plumages even in the same nesting- 

 places, one form being sooty the other having light under parts. 

 The white-breasted pair one with another, and with the dark 

 forms, which also pair together. "Both extremes, and the 

 gradations resulting from their union," says Mr. Howard 

 Saunders, " are found breeding on our northern islands, the 

 Faroes, Iceland, the coasts of Scandinavia [and] Russia." On 

 Spitzbergen, however, the dark form appears to be conspicuous 



