THE. HISTORY OF LIFE 187 



ancient sea-floors, star-fish and worms lived then. 

 Crustacea are abundantly preserved in these formations, 

 for we may include the Trilobites under this head, 

 now that they are known to possess antennae. The 

 lowest orders of shell-fish were very abundant in those 

 days. Some of the genera exist at the present day. 

 Fishes now appear in the upper layers of these rocks 

 the first evidence of vertebrated life, that is, animals 

 which have back-bones. The records of these animals, 

 however, are scanty. No traces of vertebrated land 

 animals are yet found. Scorpions lived when these 

 rocks were deposited, having a poison gland and sting 

 in the tail the same as the modern creature has. Insects 

 also lived in these times. 



The Silurian rocks are found in Britain 19,500 feet 

 thick more than three and a half miles, also in Russia, 

 Norway and Sweden, France, Central and Southern 

 Europe, North America, Asia and Australia. 



Already, then, we see that Nature's book shows a 

 distinct advance from the simple to the more complex 

 organisms, while the proportions in which the several 

 life-groups occurred in Cambrian times have markedly 

 changed by the time that we reach the top of the 

 Silurian. 



The next system of strata, or those which were 

 deposited after and upon the foregoing groups of rocks, 

 is called by geologists, the Devonian rocks. These 

 rocks merge into the Silurian rocks there is no 

 distinct boundary. On the top of the latter and re- 

 presenting the Devonian rocks are in places the red 

 rocks, known as the Old Red Sandstone, 20,000 feet 

 nearly four miles thick. These rocks in the British 

 Isles are destitute of marine life. 



