32 THE LESSON OF EVOLUTION 



although its affinities are rather uncertain, is thought 

 by Brongniart to belong to the Orthoptera. No 

 other order of insects appears at that time to have 

 been in existence. Scorpions, with stings at the 

 end of their tails, like those of the present day, and, 

 therefore, carnivorous, have also been found in the 

 Silurian, and spiders in the Carboniferous. 



SPECULATIONS ON ORDOVICIAN AND SILURIAN LIFE 



The peopling of the shore-line and the land. Pro- 

 bably all the different sub-kingdoms of animals had 

 come into existence before the close of the Cambrian 

 period. Henceforward no more fundamental types 

 were to be introduced ; multiplication and variation 

 of the existing types was for the future to be the 

 role, until all habitable parts of the earth were filled 

 with life. It is in the early part of the Ordovician 

 period that we first see animals fitted to live in the 

 rough waters of the littoral zone of the sea-shore : 

 these were thick-shelled gastropods, followed in the 

 Silurian by the Ostracodermi. It is in the Ordo- 

 vician that we have the first proofs of the existence 

 of land-plants and insects, followed in the Silurian 

 by scorpions feeding on the insects. 



Rate of Variation. When we think that certainly 

 seven, and probably eight, of the sub-kingdoms of 

 animals were in existence before the close of the 

 Cambrian period, it would seem at first that variation 

 had gone on more rapidly during the earlier periods 

 of the earth's history than afterwards ; but this is an 



