34 ANCIENT PLANTS 



meaning for us, for the human mind cannot compre- 

 hend the significance of vast numbers, of immense space, 

 or of eeons of time. Hence when we look back on 

 the history of the world we cannot attempt to give 

 even approximate dates for its events, and the best we 

 can do is to speak only of great periods as units whose 

 relative position and whose relative duration we can 

 estimate to some extent. 



Those who have studied geology, which is the 

 science of the world's history since its beginning, have 

 given names to the great epochs and to their chief sub- 

 divisions. With the smaller periods and the subdivisions 

 of the greater ones we will not concern ourselves, for our 

 study of the plants it will suffice if we recognize the main 

 sequence of past time. 



The main divisions are practically universal, and 

 evidence of their existence and of the character of the 

 creatures living in them can be found all over the 

 world; the smaller divisions, however, may often be 

 local, or only of value in one continent. To the spe- 

 cialist even the smallest of them is of importance, and 

 is a link in the chain of evidence with which he can- 

 not dispense; but we are at present concerned only with 

 the broad outlines of the history of the plants of these 

 periods, so will not trouble ourselves with unnecessary 

 details.^ Corresponding to certain marked changes in 

 the character of the vegetation, we find seven impor- 

 tant divisions of geological time which we will take as 

 our unit periods, and which are tabulated as follows : — 



^ • • f I. Present Day. 

 Cainozoic I jj_ .p^^jj^^y y 



Mesozoic ^ ^^^" Upper Cretaceous (or Chalk), 

 t IV. The rest of the Mesozoic. 



("Permian. 

 Palaeozoic ^' ^^^^'^"^ Palseozoic, including \ Carboniferous. 



(.Devonian. 

 VI. Older Palaeozoic. 

 Eozoic ... VII. Archaean. 



' For a detailed list of the strata refer to Watts, p. 219 (see Appendix). 



