476 
POPULAR SCIENCE REVIEW. 
liken to detached villas rather than to the crowded houses of a 
town. For the nests of these little birds are long purse-like 
structures, curiously woven of reeds and grass, in the form of 
long bags, with the entrance-hole at the lower end, suspended 
from the projecting boughs and twigs of some wide-spreading 
tree, generally over a river or other water. Several hundreds 
of these “ tree-rocked cradles/’ all separate, though in close 
proximity, have been observed depending from the branches of 
one tree. 
Two forms of instinct are presented to us in these examples, 
which, though often conjoined, are yet distinct. There is the 
constructive instinct displayed in strong’ development, and 
there is the instinct of association. In multitudes of cases we 
find these two tendencies acting separately. If there are social 
bees and wasps, there are solitary species too, which build 
their habitations without seeking the solace of companionship. 
The squirrel builds its little drey in the pine-tree, and the 
bottle-tit weaves its beautiful nest below him, -without the 
slightest wish on the part of either to have any companion but 
his own mate and offspring. On the other hand, there are 
numberless examples of the social habit with no special con- 
structive power. The wild dog hunts in packs ; the gannet and 
the gull congregate around the precipitous cliffs ; the tiny gnats 
play on twinkling wings, in clouds, under a December sun ; — 
but there is no particular tendency to prosecute the builder’s 
art in either. 
It is difficult to understand this tendency to associate without 
attributing to the animals which display it something analogous 
to human feelings and affections. In some cases, it is true, we 
can trace an utilitarian end more or less obvious. It is evident 
that the structure of the termites could not be built without 
concert ; the isolated beaver would be seemingly far less com- 
fortably housed and far less efficiently protected if he had not the 
advantage of the dam and house, which yet he could not achieve 
alone ; though isolated beavers, “ old bachelors,” are found on 
the European rivers. The sociable grosbeak, too, derives addi- 
tional protection from the massiveness of the thatch which the 
united powers of the colony produce, at least we may suppose 
so : but what is gained, beyond the amenities of companionship, 
by the pensile grosbeak’s choosing to hang his separate bottle- 
nest on the same tree and on the same twig as his fellow? 
The pack of wild hounds may run down a stronger and a 
swifter prey than a single dog could master ; but what of material 
prosperity does the little winter-gnat gain by dancing in com- 
pany with a thousand more, which it would lose if it had chosen 
to frisk alone a dozen yards off, in the same air and under the 
same sunbeam? 
