bution of Insects._

The marvellous development of insects into such an endless variety of
forms, their extreme specialisation, and their adaptation to almost
every possible condition of life, would almost necessarily imply an
extreme antiquity. Owing, however, to their small size, their lightness,
and their usually aerial habits, no class of animals has been so
scantily preserved in the rocks; and it is only recently that the whole
of the scattered material relating to fossil insects and their allies
have been brought together by Mr. Samuel H. Scudder of Boston, and we
have thus learned their bearing on the theory of evolution.[194]

The most striking fact which presents itself on a glance at the
distribution of fossil insects, is the completeness of the
representation of all the chief types far back in the Secondary period,
at which time many of the existing families appear to have been
perfectly differentiated. Thus in the Lias we find dragonflies
"apparently as highly specialised as to-day, no less than four tribes
being present." Of beetles we have undoubted Curculionidae from the Lias
and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
Lias. All these forms are well represented, but there are many other
families doubtfully identified in equally ancient rocks. Diptera of the
families Empidae, Asilidae, and Tipulidae have been found as far back as
the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
the Oolite; while ants, representing the highly specialised Hymenoptera,
have occurred in the Purbeck and Lias.

This remarkable identity of the families of very ancient with those of
existing insects is quite comparable with the apparently sudden
appearance of existing genera of trees in the Cretaceous epoch. In both
cases we feel certain that we must go very much farther back in order to
find the ancestral forms from which they were developed, and that at any
moment some fresh discovery may revolutionise our ideas as to the
antiquity of certain groups. Such a discovery was made while Mr.
Scudder's work was passing through the press. Up to that date all the
existing orders of true insects appeared to have originated in the
Trias, the alleged moth and beetle of the Coal formatio