xrn THE GEOLOGICAL EVIDENCES OF EVOLUTION 403 



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 respec- 

 tive grades of development, and thus offer no obstacle to the 

 acceptance of the belief in their progressive evolution. 



Geological Distribution of Insects. 



The marvellous development of insects into such an endless 

 variety of forms, their extreme specialisation, and their adapta- 

 tion 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. 1 



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 Curculionidse from the Lias and Trias ; Chrysome- 

 lidse in the same deposits ; Cerambycidae in the Oolites ; 

 Scarabaeidaa in the Lias ; Buprestidse in the Trias ; Elateridae, 

 Trogositidse, and Nitidulidas in the Lias ; Staphylinidae in the 

 English Purbecks; while Hydrophilidae, Gyrinidae, and Carabidse 

 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 Empida?, Asilida\ 

 and Tipulidse have been found as far back as the Lias. 

 Of Lepidoptera, Sphingidae and Tineidse have been found 



1 Systematic Review of our Present Knowledge of Fossil Insects, including 

 Myriapods and Arachnids (Bull, of U. S. Geol. Survey, No. 31, Washington, 

 1886). 



