When animals apply themselves in association to any work, they nearly always exhibit in it a marked superiority over neighboring species among whom the individuals work in isolation. The construction of dwellings is no exception, and the nests of the sociable weaver birds of South Africa are the best constructed that can be found. These birds live together in considerable colonies; the members of an association are at least two hundred in number, and sometimes rise to five hundred. The city which they construct is a marvel of industry. They first make with grass a sloping roof, giving it the form of a mushroom or an open umbrella, and they place it in such a way that it is supported by the trunk of a tree and one or two of the branches. This thatch is prepared with so much care that it is absolutely impenetrable to water. Beneath this protecting shelter each couple constructs its private dwelling. All the individual nests have their openings below, and they are so closely pressed against one another that, on looking at the construction from beneath, the divisions can not be seen. One only perceives a surface riddled with holes like a skimmer; each of these holes is the door of a nest. The work may endure for several years; as long as there is room beneath the roof the young form pairs near their cradle; but at last, as the colony continues to increase, a portion emigrate to found a new town on another tree in the forest. The industry of the woven dwelling does not flourish among mammals; but there is one which excels in it. This is the dwarf mouse (Mus minutus), certainly one of the smallest rodents. It generally lives amid reeds and rushes, and it is perhaps this circumstance which has impelled it to construct an aerial dwelling for its young, not being able to deposit them on the damp and often flooded soil. This retreat is not used in every season; its sole object is for bringing forth the young. It is therefore a genuine nest, not only by the manner in which it is made, but by the object it is intended to serve. The nest is made with as much delicacy as that of any bird, and no other mammal except man is capable of executing such weaver’s work.