270 HIGH SCHOOL ZOOLOGY. 



V 



forms of plants and animals are the modified descendants of 

 other species now extinct) a continuous development from 

 lower to higher forms, presenting in its entirety what is 

 often called the evolution of organic Nature. It is, in fact> 

 possible to subdivide the sedimentary rocks according to 

 the prevailing kinds of life during their deposition, into 

 Archaean — which contain few, if any, traces of life — and Palaeo- 

 zoic, Mesozoic, and Kainozo c, in all of which fossils are abun- 

 dant, but the last of which alone contain any resembling the 

 forms of life at px'esent on the earth. Some information may be 

 gleaned from the accompanying Table as to the characteristic 

 plants and animals of tliese various periods, and as to the order 

 of their introduction. Not only is the appearance of the life of 

 these periods charactei'istic, but the classification is assisted, in 

 Europe at least, by the occurrence of conspicuous ' breaks ' 

 between them, which indicate, for example, that the Palaeozoic 

 strata were elevated into dry land, partly denuded and 

 upheaved, before they again sank, and had the Mesozoic strata 

 laid down upon them unconformahly — i.e. not in plains paral- 

 lel to those below as if they had been deposited consecutively. 

 Such breaks do not, of coui'se, occur at the same ' horizon ' in 

 other parts of the world, but the classification from the pre- 

 vailing kinds of life answers all over the world. Each age is 

 again divisible into various epochs, which are marked by moi'e 

 or less distinct breaks in different places, and also by charac- 

 teristic fossils, so that the geologist is enabled to recognize the 

 rocks of any particular horizon by diagnosing the contained 

 fossils. 



10. An important question which comes up in connection 

 with these strata, is that of their age, and, consequently, that 

 of the contained fossils. If we assume that the rate of deposi- 

 tion has been uniform throughout the various strata, then the 

 maximum thickness may be taken as a measure of the relative 

 duration of the time during which they were deposited. The 



