have no monopoly of these careful dwellings; a
considerable number of genera have carried this industry to the same
degree of perfection.

[Illustration: FIG. 28.]

[Illustration: FIG. 29.]

When animals apply themselves in association to any work, they nearly
always exhibit in it a marked superiority over neighbouring 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.
(Fig. 29.) 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 cannot 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.[97]

[97] An early description of this bird is to be found in W.
Paterson's _Narrative of Four Journeys into the Country of
the Hottentots_, 1789; also in Le Vaillant's _Second Voyage
dans l'interieur de l'Afrique_, 1803, t. iii., p. 322.

[Illustration: FIG. 30.]

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
amidst 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 genuin