st laborious of the
whole process. As the capillary channels are prepared in advance a
slight injection of fluid is sufficient to cause expansion.

But what were these four bundles of tissue while still enclosed in their
sheaths? Are the wing-sheaths and the triangular winglets of the larva
the moulds whose folds, wrinkles, and sinuosities form their contents in
their own image, and so weave the network of the future wings and
wing-covers?

Were they really moulds we might for a moment be satisfied. We might
tell ourselves: It is quite a simple matter that the thing moulded
should conform to the cavity of the mould. But the simplicity is only
apparent, for the mould in its turn must somewhere derive the requisite
and inextricable complexity. We need not go so far back; we should only
be in darkness. Let us keep to the observable facts.

I examine with a magnifying-glass one of the triangular coat-tails of a
larva on the point of transformation. I see a bundle of moderately
strong nervures radiating fan-wise. I see other nervures in the
intervals, pale and very fine. Finally, still more delicate, and running
transversely, a number of very short nervures complete the pattern.

Certainly this resembles a rough sketch of the future wing-case; but
how different from the mature structure! The disposition of the
radiating nervures, the skeleton of the structure, is not at all the
same; the network formed by the cross-nervures gives no idea whatever of
the complex final arrangement. The rudimentary is succeeded by the
infinitely complex; the clumsy by the infinitely perfect, and the same
is true of the sheath of the wing and the final condition of its
contents, the perfect wing.

It is perfectly evident, when we have the preparatory as well as the
final condition of the wing before our eyes, that the wing-sheath of the
larva is not a simple mould which elaborates the tissue enclosed in its
own image and fashions the wing after the complexities of its own
cavity.

The future wing is not contained in the sheath as a bundle, which will
astonish us, when expanded, by the extent and extreme complication of
its surface. Or, to speak more exactly, it is there, but in a potential
state. Before becoming an actual thing it is a virtual thing which is
not yet, but is capable of becoming. It is there as the oak is inside
the acorn.

A fine transparent cushion limits the free edge of the embryo wing and
the embryo wing-case. Under