A case placed to the north of the one containing Fowls illustrates a remarkable instance of external differences in the two sexes and changes in plumage at different seasons, not under the influence of domestication. The birds in it belong to one species, the Ruff (_Pavoncella_, or _Machetes_, _pugnax_), of which the female is called Reeve; a member of the Plover family (_Charadriidæ_). In the upper division of the case are shown the eggs, newly-hatched young, and young males and females in the first autumn plumage; as well as adult males and females in winter, when both sexes are exactly alike in colour and distinguishable externally by size alone. The large group occupying the lower part of this case consists of adult birds in the plumage assumed in the breeding time (May and June). In the female the only alteration from the winter state is a darker and richer colouring, 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 wild animals, may be observed, namely, striking diversity of colour in different individuals. Of the twenty-three specimens shown no two are entirely alike. [Sidenote: Wild Ducks illustrating Seasonal Change of Plumage.] Next in order stands a case displaying the variations, according to season and age, in the plumage of the Wild Duck, or Mallard (_Anas boscas_). The most noticeable feature in the plumage-changes is the assumption in summer by males of an “eclipse-plumage,” resembling the one worn by females at all seasons. At other times the males are more brilliantly coloured than their partners. The eclipse-plumage corresponds to the winter, or non-breeding, dress of other birds which have a seasonal change. [Sidenote: Adaptation of Colour to surrounding Conditions.]