36 THE LESSON OF EVOLUTION 



which no doubt contained numerous organic par- 

 ticles. So again we must have recourse to 

 predaceous foes. This reasoning is much 

 strengthened by the fact that in Ordovician and 

 Silurian times the Trilobites had learnt how to 

 defend themselves by rolling themselves up, a feat 

 which the Cambrian Trilobites were not able to 

 perform. Now the earliest powerful predaceous 

 animals we know were the ground Cephalopods, 

 which first appearing in the Upper Cambrian rapidly 

 increased in importance during the Ordovician, 

 and especially during the Silurian ; the 

 relative numbers of the Nautiloidea in these three 

 periods being as 1 : 9 : 33. In the Cambrian and 

 Ordovician periods the Trilobites had greatly in- 

 creased, but in the Silurian they began to decline in 

 numbers, and rapidly diminished during the 

 Devonian and Carboniferous, although a few lin- 

 gered on to the Permian. This decline of the 

 Trilobites coincides in time with the expan- 

 sion of the Nautiloidea, and was, I have little doubt, 

 caused by it. These ravenous Cephalopods, the 

 precursors of our gigantic cuttle-fish, were the 

 earliest rovers of the sea. Some lived near the 

 surface and fed on Graptolites. Others sank to the 

 bottom, where the inoffensive Trilobites had reigned 

 for ages undisturbed, quietly sucking mud. But the 

 ruthless intruders turned the Trilobites over and tore 

 out their insides , in spite of their attempts to defend 

 themselves by rolling up into a ball. 



