xi i r 
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- 
lidae in the same deposits; Cerambycidae in the Oolites; 
Scarabseidse in the Lias ; Buprestidie in the Trias ; Elateridse, 
Trogositidae, and Nitidulidse in the Lias ; Staphylinidae in the 
English Purbecks; while Hydrophilidae, Gyrinidse, 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 
1 Systematic Review of our Present Knowledge of Fossil Insects, including 
Myriapods and Arachnids (Bull, of U. S. Geol. Survey, No. 31, Washington, 
1886). 
