the ladder of the water weeds. The average density of
the sheath is of no importance, so long as the burden to be dragged is
not beyond the animal's strength. Besides, the weight of the load is
greatly reduced when moved in the water.

The admission of a bubble of air into the back chamber, which the animal
ceases to occupy, allow it, without further to-do, to remain for an
indefinite period on the surface. To dive down again, the caddis worm
has only to retreat entirely into its sheath. The air is driven out; and
the canoe, resuming its mean density, a greater specific density than
that of water, goes under at once and descends of its own accord.

There is, therefore, no choice of materials on the builder's part, no
nice calculation of equilibrium, save for one condition, that no stony
matter be admitted. That apart, everything serves, large and small,
joist and shell, seed and billet. Built up at haphazard, all these
things make an impregnable wall. One point alone is essential: the
weight of the whole must slightly exceed that of the water displaced; if
not, there could be no steadiness at the bottom of the pond, without
a perpetual anchorage struggling against the pull of the water. In the
same manner, quick submersion would be impossible at times when the
surface became dangerous and the frightened creature wanted to leave it.

Nor does this important heavier-than-water question call for lucid
discernment, seeing that almost the whole of the sheath is constructed
at the bottom of the pond, whither all the materials picked up at
random, having descended once before, are likely to descend again. In
the sheaths, the parts capable of floating are very rare. Without taking
their specific levity into account, simply so as not to remain idle, the
caddis worm fixed them to its bundle when sporting on the surface of the
water.

We have our submarines, in which hydraulic ingenuity displays its
highest resources. The caddis worms have theirs, which emerge, float on
the surface, dip down and even stop at mid-depth by releasing gradually
their surplus air. And this apparatus, so perfectly balanced, so
skilful, requires no knowledge on the part of its constructor. It comes
into being of itself, in accordance with the plans of the universal
harmony of things.

CHAPTER IX. THE GREENBOTTLES

I have wished for a few things in my life, none of them capable of
interfering with the common weal. I have longed to possess a po