The Story of Evolution 91 



The rarity of direct traces of life in the oldest rocks is partly 

 due to the fact that the primitive animals would be of delicate 

 build, but it must also be remembered that the ancient rocks have 

 been profoundly and repeatedly changed by pressure and heat, so 

 that the traces which did exist would be very liable to obliteration. 

 And if it be asked what right we have to suppose the presence 

 of living creatures in the absence or extreme rarity of fossils, we 

 must point to great accumulations of limestone which indicate the 

 existence of calcareous alga?, and to deposits of iron which prob- 

 ably indicate the activity of iron-forming Bacteria. Ancient beds 

 of graphite similarly suggest that green plants flourished in these 

 ancient days. 



3 



The Era of Ancient Life (Palaeozoic) 



The Cambrian period was the time of the establishment of 

 the chief stocks of backboneless animals such as sponges, jelly- 

 fishes, worms, sea-cucumbers, lamp-shells, trilobites, crustaceans, 

 and molluscs. There is something very eloquent in the broad fact 

 that the peopling of the seas had definitely begun some thirty mil- 

 lion years ago, for Professor H. F. O shorn points out that in the 

 Cambrian period there was already a colonisation of the shore of 

 the sea, the open sea, and the deep waters. 



The Ordovician period was marked by abundant represen- 

 tation of the once very successful class of Trilobites jointed- 

 footed, antenna-bearing, segmented marine animals, with 

 numerous appendages and a covering of chitin. They died away 

 entirely with the end of the Palaeozoic era. Also very notable was 

 the abundance of predatory cuttlefishes, the bullies of the ancient 

 seas. But it was in this period that the first backboned animals 

 made their appearance an epoch-making step in evolution. In 

 other words, true fishes were evolved destined in the course of 

 ages to replace the cuttlefishes (which are mere molluscs) in 

 dominating the seas. 



