On the same side of the hall follow two cases illustrating the adaptation of the colour of animals to their natural surroundings, by means of which they are rendered less conspicuous 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, together with some Willow-Grouse and Ptarmigan, as well as an Arctic Fox, in their summer dresses. All were obtained in Norway, and show the general harmony of their colouring at this season with that of the rocks and plants among which they live. The second case displays examples of the same animals obtained from the same country in winter, when the ground was covered with snow. Such striking 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 Variable Hare in Ireland. A few permanent inhabitants of still more northern regions, where the snow remains throughout the year, such as the Polar Bear, Alaskan Bighorn Sheep, Greenland Falcon, and Snowy Owl, retain the white dress throughout the year. The whiteness of these animals must not be confounded with _albinism_, or whiteness occurring in individuals of species normally of a different colour, which is illustrated in a case on the other side of the hall. [Sidenote: Protective Resemblance of Desert Animals to their Surroundings.] The case on the east side of the hall nearest the great staircase contains examples of conformity of general style of colouring to surrounding conditions, as exemplified by some of the commoner Birds, Mammals, and Reptiles of the Egyptian desert, placed on the stones and sand amid which they habitually dwell. The advantage of this colouring in concealing the herbivorous species from their enemies, and in enabling the carnivorous to approach their prey unperceived, is obvious. Many excellent cases of concealment by adaptation to surroundings, especially in eggs and young Birds, may be seen among the groups in the Bird-gallery. [Sidenote: Mimicry.]