Unfortunately, in spite of the amount of land on this continent, it is difficult for any but the very wealthy to have access to much of it, hence many readers will say, “We have no yard in which to keep pets,” or, “Our yard is too small.” Of course, if you are living in a flat you must go without a zoo, but if you have a yard it will probably not be less than twenty-five feet wide, and Fig. 22 shows how a very comprehensive zoo can be placed in the rear of a twenty-five foot lot, without materially interfering with such domestic matters as the drying of clothes on wash-day. A city [Illustration: FIG. 22.--Arrangement of Dens in the Back-yard.] Lot Twenty-five Feet Wide is usually one hundred feet deep; this will allow plenty of room for the house and the clothes-lines, and still leave the end of the lot for a famous back-yard zoölogical garden. When the writer was building back-yard zoos on the banks of the muddy Licking River, in old Kentucky, wire-cloth and wire-netting were unknown, and a few old barrels and dry-goods boxes, a saw, hatchet, and some nails, constituted the materials and tools with which he and his playmates made cages for pets, frog-ponds, and dove-cots. The writer’s Crow and Dog did the Bossing of the work, and incidentally learned all the weak spots in the structures, a knowledge which they were not slow to use when the sheds and coops were finished, and occupied by creatures fascinatingly interesting to crows and dogs. But you boys are lucky fellows! Everything that youth wants is now on the market at reasonable prices. Wire-cloth and galvanized wire-netting with double-twisted selvages, with meshes of any size to suit the occasion, and wire of any dimensions to suit the purpose, are now manufactured especially for the building of cages. Galvanized Iron Wire-Cloth or Netting comes in rolls, with either square or hexagonal mesh; in other words, the openings between the wires are in the form of a square, or are six-sided. [Illustration: FIG. 23.--Temporary Frame of Cage.] Wire is numbered from the very heavy, No. 000, which is over a third of an inch in diameter, to No. 40, which is only .00725 of an inch in diameter. It is not very likely that you will use either of these wires, unless your collection includes some very large and strong beasts and some very small insects. The wires which you will probably need will be between No. 14 and No. 22. No. 14 is eighth-tenths of an inch in diameter, and No. 22 is a little more than two-tenths of an inch in diameter. The Mesh is the distance from _centre to centre_ of the wire. No. 5 mesh means five meshes to the lineal inch--that is, a piece of netting five inches long will contain twenty-five meshes. The “space” means the opening between the wires--that is, the distance from _wire to wire_.