THE PROGRESS OF LIFE 67 



Land Animals. lu the Ordovician of Sweden the 

 remains of an insect (Protocimex), belonging to the 

 Hemiptera, has been found. And in the Silurian of 

 France there is PalceoUattina douvillei, which, although 

 its affinities are rather uncertain, is thought by Brong- 

 niart 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 Ordovician 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 all eight, of the sub-kingdoms of 

 animals were in existence before the close of the 

 Cambrian period, it would seem at first that variation 



