Page:Darwinism by Alfred Wallace 1889.djvu/427

 The shaded vertical bands exhibit the proportions of the fossil forms actually discovered, while the outline extensions are intended to show what we may fairly presume to have been the approximate periods of origin, and progressive increase of the number of species, of the chief divisions of the vegetable kingdom. These seem to accord fairly well with their respective grades of development, and thus offer no obstacle to the acceptance of the belief in their progressive evolution.

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.

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