Still it may safely be inferred that when the structure of
these remains clearly indicates affinity to some existing order or
family, the life-history of the extinct creature must have resembled, on
the whole, that of its nearest living allies. And all the fossil
insects known can be either referred to existing orders, or shown to
indicate definite relationship to some existing group.

Passing over some doubtful remains of Silurian age, we find in rocks
usually regarded as Devonian[13] the most ancient fossils that can be
certainly referred to the insects, while from beds of the succeeding
Carboniferous period, a number of insect remains have been disinterred.
These Palaeozoic insects were frequently of large size, and they show
distinct affinities with our recent may-flies, dragon-flies,
stone-flies, and cockroaches. In the Permian period, the latest of the
divisions of the Palaeozoic, lived Eugereon, an insect with hemipteroid
jaws and orthopteroid wings. All these insects must have been
exopterygote in their life-history, if we may trust the indications of
affinity furnished by their structure. In the Mesozoic period, however,
insects with complete transformations must have been fairly abundant.
Rocks of Triassic age have yielded beetles and lacewing-flies, while
from among Jurassic fossils specimens have been described as
representing most of our existing orders, including Lepidoptera,
Hymenoptera and Diptera. In Cainozoic rocks fossil insects of nearly six
thousand species have been found, which are easily referable to
existing families and often to existing genera. We may conclude then,
imperfect though our knowledge of extinct insects is, that some of the
most complex of insect life-stories were being worked out before the
dawn of the Cainozoic era. Some instructive hints as to differences in
the rate of change among different insect groups may be drawn from the
study of parasites. For example, V.L. Kellogg (1913) points out that an
identical species of the Mallophaga (Bird-lice) infests an Australian
Cassowary and two of the South American Rheas; while two species of the
same genus (Lipeurus) are common to the African Ostrich and a third kind
of South American Rhea. These parasites must have been inherited
unchanged by the various members of these three families of flightless
birds from their common ancestors, that is from early Cainozoic times at
latest. On the other hand, the various kinds of such highly speci