186 WHAT IS LIFE? 



deposited. Worms are traced by their frequent trails 

 and burrows. The most abundantly preserved forms 

 of life are crustaceans, that is, animals having their 

 skeleton outside, like lobsters and crabs here repre- 

 sented by the trilobites. Brachiopods, a humble class of 

 shell-fish, existed in abundance. Some of the latter, 

 with but little change, have existed from this vastly 

 remote period to the present day. 



All these animals were built up of cells. All the 

 cells were living creatures as now. The shells of the 

 shell-fish and trilobites were formed by the mass of 

 cells collectively called the living fish. All past life is 

 like the present, but not so complex. The process was 

 the same, the birth, growth, and final death of each 

 individual took place by the same inexorable laws as 

 exist to-day, each organism leaving the shell and the 

 hard substances to be the geological record of the 

 history of the individual. 



The Cambrian rocks are found in Great Britain, 

 Norway, Sweden, Central Europe, North America, 

 South America, India and Australia. 



The next great system of strata lying over and upon 

 the Cambrian strata, and subsequently deposited, is a 

 series of rocks called by the geologist the Silurian 

 rocks. Again there is no hard and fast line, the 

 Silurian rocks merge into the Cambrian rocks. Sea- 

 weeds are often beautifully preserved in these rocks. 

 Traces of land plants are found. 



Foraminifera, that is, minute shells secreted by the 

 simplest form of life protoplasm, existed, some of 

 which have passed through the whole of subsequent 

 geological time to the present day. Sponges grew in 

 these ancient seas. Corals swarmed in parts of the 



