tyle, neatness, and finish of the nests, as well as
in the materials used. Skeins of silk and hanks of thread have
frequently been found in the Baltimore Bird's nest, so woven up and
entangled that they could not be withdrawn. As such materials could
not be obtained before the introduction of Europeans, it is evident
that this bird, with the sagacity of a good architect, knows how to
select the strongest and best materials for his work. Many other facts
might be quoted, but these suffice to show that the species is not
animated by an inevitable instinct, but that each individual, skilful
no doubt by heredity, can modify the methods transmitted to him by his
ancestors, according to his own experience and his own judgment.

_Built dwellings._--The built dwelling, the expression of the highest
civilisation, still remains to be studied. Man has only known how to
construct this kind of shelter at a comparatively late period in his
evolution; and among animals we do not find it widely spread, much
less so, certainly, than the two foregoing methods, especially the
first. The difficulty of this work is greater, and it only arrives at
considerable development among very sociable species, since the united
efforts of a great number of individuals are needed to carry it on.

There are, however, masons who operate separately; but their
constructions are rudimentary. The characteristic of all these works
is that they are manufactured with some substance to which the animal
gives a determined form while it is still soft, and that in drying it
preserves this form and acquires solidity. The matter most usually
employed is softened and tempered earth--mortar; but there are animals
who use with success more delicate bodies. Two examples will suffice
to indicate the nature of these exceptions: the labours of Wasps and
those of certain Swallows.

[Illustration: FIG. 33.]

_Paper nests._--Certain Wasps, by the material of their dwellings,
approach the Japanese; they build with paper. This paper or cardboard
is very strong and supplies a solid support; moreover, being a bad
conductor of heat, it contributes to maintain an equable temperature
within the nest. The constructions of these insects, though they do
not exhibit the geometric arrangement of those of Bees, are not less
interesting. The paper which they employ is manufactured on the spot,
as the walls of the cells develop. Detritus of every kind enters into
its preparation: small fr