Animals Before Man 



form a bed of rock a foot thick and even a 

 mile square. 



As witli vertebrates, all orders of inverte- 

 brates now living are rej)resented by fossils, 

 except where they have been too soft and small 

 for preservation, while several important orders 

 occur only as fossils, and several more are even 

 now verging on extinction. 



Owing to their hard coverings, the shell- 

 bearing mollusks are naturally the most abun- 

 dant of fossils, and many thousand species are 

 known. They play a most important part in 

 defining the limits of groups of rocks and in 

 identifying the individual beds, some species 

 being found through a number of strata, while 

 again others are confined to and characteristic 

 of a single layer of rock. And this furnishes a 

 hint of the intimate connection there is between 

 the divisions of life and time and of the manner 

 in which the latter are defined. 



There is a ceaseless warfare waged by water 

 against the land ; the sea hurls its waves against 

 the coast, rivers cut their channels through 

 earth and rocks, and eveiy rain washes some- 



52 



