VALUE OF PROTECTIVE COLOURINGS. 325 



tective colouring collected by Mr. F. S. Worthington, 

 in 1890, from the Egyptian deserts are all sand 

 coloured, to match the colouring of the surrounding 

 landscape while the white birds, and small rodents, 

 illustrative of the same subject in the arctic zone, 

 were presented by Professor Collett, of the Zoological 

 Museum at Christiania, in 1891. Only in three instances, 

 we are informed by the descriptive cards attached to 

 these cases, is the white colour retained throughout 

 the year namely in those of the snowy owl (Strix 

 Nyctea [Linn.]), the Greenland falcon (Falco Candi- 

 cans [Linn.]) and the polar bear (Ursus Maritimus}. 

 The summer livery of all the others conforms to the 

 hue of their surroundings at that season, and the white 

 dress is assumed only during winter. Among the 

 latter the ptarmigan in spring is found to have its 

 plumage mottled with brown spots and bars. This is 

 usually regarded as an effort of Nature to preserve 

 them from the attacks of enemies; and it must be 

 manifest that snow-white birds among brown rocks 

 and gravels would be very conspicuous objects in 

 summer time; the very great value of the protection 

 afforded by the brown dress thus at once becomes 

 apparent: but if it prevailed in winter time it would 

 then be disastrous. The prevalence during winter 

 time of white in the colourings of ptarmigan and other 

 arctic creatures, is therefore to be regarded as the 

 result of a positive natural law, " the ultimate survi- 

 val of the fittest," which Mr. Darwin first brought 

 into prominent notice. It is evident that brown birds 

 or animals would as we have observed become such 

 conspicuous objects on the surface of a snow field, 

 that they would certainly stand much less chance of 



