152 ELEMENTS OF BIOLOGY. 



into a number of definite groups of strata, \vhich are 

 technically called " formations." 



5. Each of these definite rock-groups, or " formations," is 

 characterised by the occurrence of an assemblage of fossil 

 remains more or less peculiar and confined to itself. 



6. The majority of these fossil forms are " extinct ;" that 

 is to say, they do not admit of being referred to any species 

 at present existing. 



7. No fossil, however, is known, which cannot be referred 

 to one or other of the primary subdivisions of the Animal 

 Kingdom, which are represented at the present day. 



8. When a species has once died out it never reappears. 



9. The older the formation, the greater is the divergence 

 between its fossils and the animals and plants now existing 

 on the globe. 



10. All the known formations are divided into three great 

 groups, termed respectively Palaeozoic or Primary, Mesozoic 

 or Secondary, and Kainozoic or Tertiary. 



The Palaeozoic or Ancient-life period is the oldest, and 

 is characterised by the marked divergence of the life of the 

 period from all existing forms. 



In the Mesozoic or Middle-life period, the general fades 

 of the fossils approaches more nearly to that of our existing 

 fauna and flora; but with very few exceptions the 

 characteristic fossils are all specifically distinct from all exist- 

 ing forms. 



In the Kainozoic or New-life period, the approximation 

 of the fossil remains to existing living beings is still closer, 

 and some of the forms are now specifically identical with 

 recent species ; the number of these increasing rapidly as 

 we ascend from the lowest Kainozoic deposit to the Recent 

 period. 



