THE PROGRESS OF LIFE 71 



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 Silu- 

 rian ; 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 increased, but in the Silurian they began to 

 decline in numbers, and rapidly diminished during the 

 Devonian and Carboniferous, although a few lingered 

 on to the Permian. This decline of the Trilobites co- 

 incides in time with the expansion 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. 



SUMMARY 



We have thus arrived at the conclusion that the 

 ocean was the mother of life ; that on its surface 

 floated the first organisms, whose descendants, but little 

 changed during all the millions of years that have 

 since past away, still float and multiply. Presently 

 some of these animals found their way down to the 

 bottom, where all the debris from the floating organ- 



