woodlice. But among the Insects, a class of predominantly terrestrial
and aerial creatures producing large eggs, the highest groups undergo,
as we shall see, the most profound changes. The life-story of the
butterfly, then, well-known as it may be, furnishes a puzzling exception
to some wide-reaching generalisations concerning animal development. And
the student of science often finds that an exception to some rule is the
key to a problem of the highest interest.

During many centuries naturalists have bent their energies to explain
the difficulties presented by insect transformations. Aristotle, the
first serious student of organised beings whose writings have been
preserved for us, and William Harvey, the famous demonstrator of the
mammalian blood circulation two thousand years later, agreed in
regarding the pupa as a second egg. The egg laid by a butterfly had not,
according to Harvey, enough store of food to provide for the building-up
of a complex organism like the parent; only the imperfect larva could be
produced from it. The larva was regarded as feeding voraciously for the
purpose of acquiring a large store of nutritive material, after which it
was believed to revert to the state of a second but far larger egg, the
pupa, from which the winged insect could take origin. Others again,
following de Réaumur (1734), have speculated whether the development of
pupa within larva, and of winged insect within pupa might not be
explained as abnormal births. But a comparison of the transformation of
butterflies with simpler insect life-stories will convince the enquirer
that no such heroic theories as these are necessary. It will be realised
that even the most profound transformation among insects can be
explained as a special case of growth.