68 THE LESSON OF EVOLUTION 



had gone on more rapidly during the earlier periods of 

 the earth's history than afterwards ; but this is an 

 erroneous impression, due to the very unequal lengths 

 of time represented by the different periods. Making 

 every allowance for the possibility that the rates of 

 denudation and deposition may have been greater in 

 past times than now, still we must admit that the 

 relative thickness of the sedimentary rocks of each 

 period is a rough measure of the relative length of 

 time it represents ; and I suppose that every geologist 

 will agree that the Huronian, the Algonkian, the 

 Cambrian, and the Ordovician were collectively at 

 least equal in duration to all the periods that came 

 after them that is, they represent at least one-half 

 of the time since life first appeared on the earth. 

 But certainly the changes which have taken place in 

 animals, and especially in plants, since the commence- 

 ment of the Silurian period, are far greater than those 

 that went before, both in the addition of new groups 

 and in the extinction of old ones ; so that the rate of 

 variation must have increased and not diminished with 

 time. It was this slow rate of variation in ancient 

 times that enabled the early Palaeozoic genera to spread 

 so much more widely over the earth than do the 

 genera of the present day. 



Extinction of Groups. The diminution or decay of a 

 whole group of animals h'rst began with the Graptolites 

 in the Upper Ordovician, and they finally became 

 extinct in the Carboniferous. The same process 

 commenced with the Trilobites in the Silurian period, 

 and they became extinct in the Permian. Can we 

 trace any cause for this gradual process of decline in 

 numbers ? The existence, in the earliest times, of 



