The anatomy of the foot is fairly well shown in the track--the insignificant thumb and the tubercles on the soles. The placing of the fore feet, one behind the other, indicates that the creature cannot climb a tree. The tail seldom or never shows. The original of this was in fine dust. The small tracks to the right show the style usually seen. There are many species of grasshopper mouse, but the tracks are not distinguishable from each other. The exact species is determined by locality, size, etc. (see pages 520 and 527).] At night the curiosity of flying squirrels about strange things and their mischievous activities are often most entertaining, and sometimes exasperating. Whatever is accessible within their territory is certain to be thoroughly explored. A large apartment building, seven stories high, in Washington stands on the border of the woods of the Zoological Park. During one summer night a friend occupying an apartment on the seventh floor of this building, fronting the park, observed some movement on one of his window sills and by later observation and by inquiry among the other residents learned that flying squirrels were habitually climbing all about the high walls to the top of this building, using it and some of the rooms as a nightly playground. Several occupants of apartments in different parts of the building regularly placed nuts of various kinds on the window ledges for them, and now and then were amused to find that during the night the squirrels had carried away some of their nuts, but had replaced them with other kinds, sometimes brought from a window at a considerable distance on another side of the building. The presence of these squirrels was warmly welcomed and furnished much interest to their hosts. The constant activity of these little animals at night enables owls and cats to capture many, but their small size and the shelter of their homes by day will prevent their serious decrease in numbers so long as suitable forests remain to supply their needs. =THE BLACK-FOOTED FERRET= (=Mustela nigripes= and its relatives) (_For illustration, see page 551_) Of all the varied forms of mammalian life in America, the black-footed ferret has always impressed me as one of the strangest and most like a stranded exotic. It is about the size of a mink, but, as the illustration shows, is entirely different in appearance and has the general form of a giant weasel. It has no close relative in America, but bears an extraordinarily close resemblance in size, form, and color to the Siberian ferret (_Mustela eversmanni_).