ADAPTATION TO SURROUNDINGS. 
11 
sexes are exactly alike in colour, size alone distinguishing 
tliem. The large group occupying the lower part of this case 
consists of adult birds in the plumage they assume in the breed- 
ing time (May and June). In the female the only alteration 
from the winter state is a darker and richer coloration, but in 
the males there is a special growth of elongated feathers about 
the head and neck, constituting the ruff," from which the bird 
derives its name. In addition to this peculiarity, another, 
rare among animals in a wild state (though so common among 
domesticated breeds), may be observed, that of striking diversity 
of colour in different individuals. Of the twenty-three specimens 
shown in the case no two are entirely alike. 
Next in order stands a case displaying the remarkable Group of Wild 
variations, according to season and age, in the plumage of the frating^ea- 
Wild Duck, or Mallard {Anas hoscas). The most noticeable sonal change 
feature in the plumage-changes of this group of birds is the °^ ^l^^^^&e. 
assumption in summer by the males of certain species of an 
''eclipse plumage," closely resembling the one worn by the 
females at all seasons. At other times the males are much 
more brilliantly coloured than their partners. 
On the same side of the hall follow two cases illustrating Cases illus- 
the adaptation of the colour of animals to their natural sur- ^^ap^f^ion of 
roundings, by means of which they are rendered less con- Colour to 
spicuous to their enemies or their prey. The first contains a 
specimen of a Mountain or Variable Hare (the common species 
of the north of Europe), a Stoat, and a Weasel, and some 
Willow Grouse and Ptarmigan, as well as an Arctic Fox, 
in their summer dress, all obtained in Norway, showing the , 
general harmony of their coloration at this season to that of 
the rocks and plants among which they live. The second case 
shows examples of the same animals obtained from the same 
country in winter, when the ground was covered with snow. 
Such complete changes as these only occur in latitudes and 
localities where the differences between the general external 
conditions in the different seasons are extreme, where the snow 
disappears in summer and remains on the ground during most 
of the winter. Even some of the species here shown do not 
habitually turn white in the less severe winters of their southern 
range, as the Stoat in England and the Hare in Ireland. In a 
