to form an
efficacious protection, utilise it to weld together and unite around
them neighbouring substances, grains of sand, fragments of shell, etc.
They thus construct a case which both resembles formations by special
organs and manufacture by the aid of foreign materials. The larvae of
_Phryganea_, who lead an aquatic life, use this method to separate
themselves from the world and prepare tubes in which to dwell. (Fig.
18.) All the fragments carried down by the stream are good for their
labours on condition only that they are denser than the water. They
take possession of fragments of aquatic leaves, and little fragments
of wood which have been sufficiently long in the water to have
thoroughly imbibed it and so become heavy enough to keep themselves at
the bottom, or at least to prevent them from floating to the surface.
It is the larva of _Phryganea striata_ which has been best studied;
those of neighbouring species evidently act much in the same way, with
differences only in detail. The little carpenter stops a fragment
rather longer than his own body, lies on it and brings it in contact
with other pieces along his own sides. He thus obtains the skeleton of
a cylinder. The largest holes are filled up with detritus of all
kinds. Then these materials are agglutinated by a special secretion.
The larva overlays the interior of its tube with a covering of soft
silk which renders the cylinder watertight and consolidates the
earlier labours. The insect is thus in possession of a safe retreat.
Resembling some piece of rubbish, it completes its metamorphosis in
peace, undisturbed by the carnivora of the stream. There is here
already a tendency towards the dwellings of which I shall speak later
on, and which are entirely formed of the external environment.

_Animals who establish their home in the natural or artificial
dwellings of others._--Between the beings whom nature has endowed with
a shelter and those who construct it by their own industry, we may
intercept those who, deprived of a natural asylum and not having the
inclination or the power to make one, utilise the dwellings of others,
either when the latter still inhabit them, or when they are empty on
account of the death or departure of the owner. In the natural
sciences there is no group of facts around which may be traced a clear
boundary; each of them is more or less closely related to a group
which appears at first of an entirely different nature. Thus it does