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THE GEOLOGICAL EVIDENCES OF EVOLUTION 
405 
from the Middle Silurian sandstone of France. If this 
occurrence of a true hexapod insect from the Middle Silurian 
he really established, taken in connection with the well- 
defined Coleoptera from the Carboniferous, the origin of the 
entire group of terrestrial arthropoda is necessarily thrown 
back into the Cambrian epoch, if not earlier. And this cannot 
be considered improbable in view of the highly differentiated 
land plants—ferns, equisetums, and lycopods—in the Middle or 
Lower Silurian, and even a conifer (Cordaites Robbii) in the 
Upper Silurian; while the beds of graphite in the Laurentian 
were probably formed from terrestrial vegetation. 
On the whole, then, we may affirm that, although the 
geological record of the insect life of the earth is exceptionally 
imperfect, it yet decidedly supports the evolution hypothesis. 
The most specialised order, Lepidoptera, is the most recent, 
only dating back to the Oolite ; the Hymenoptera, Diptera, 
and Homoptera go as far as the Lias ; while the Orthoptera 
and Neuroptera extend to the Trias. The recent discovery of 
Coleoptera in the Carboniferous shows, however, that the 
preceding limits are not absolute, and will probably soon be 
overpassed. Only the more generalised ancestral forms of 
winged insects have been traced back to Silurian time, and 
along with them the less highly organised scorpions ; facts 
Avhich serve to show us the extreme imperfection of our 
knowledge, and indicate possibilities of a world of terrestrial 
life in the remotest Palaeozoic times. 
Geological Succession of Vertebrata. 
The lowestforms of vertebrates are the fishes, and these appear 
first in the geological record in the Upper Silurian formation. 
The most ancient known fish is a Pteraspis, one of the buck¬ 
lered ganoids or plated fishes—by no means a very low type 
—allied to the sturgeon (Accipenser) and alligator - gar 
(Lepidosteus), but, as a group, now nearly extinct. Almost 
equally ancient are the sharks, which under various forms 
still abound in our seas. AYe cannot suppose these to be nearly 
the earliest fishes, especially as the two lowest orders, now 
represented by the Amphioxus or lancelet and the lampreys, 
have not yet been found fossil. The ganoids were greatly 
developed in the Devonian era, and continued till the 
