50 LIFE ON THE EARTH. 



But how many soever of centuries we allow for 

 the accumulation of the gravel, the filling of the 

 caves, or the deposition of the alluvium, this period 

 thus indicated is as nothing compared to those which 

 have passed away before it began. In a general sum- 

 mary we may affirm that the human period of the 

 earth's history is a very short one on the geological 

 scale, the latest of all, and yet the most important, 

 since independent of the interest conferred on it by 

 the presence of our race, it is by evidence collected 

 from this period that we are to judge of the earlier 

 ages of nature. 



If we put as our second question the geological 

 antiquity of the races of plants and animals which 

 are directly and specially associated with man, as the 

 valuable Pomaceous Plants and Ruminant Animals, 

 the answer is of the same kind. They are of recent 

 date their remains are found only in deposits near 

 the surface, which belong to the existing order of 

 physical conditions, the later effects of geological 

 agencies. The creatures most useful to him appear 

 to be of contemporaneous origin ; and may be em- 

 ployed as marks of the human period, in cases where 

 no traces of man or his works remain. The relation 

 of this to earlier periods will appear in the following 

 scale of geological time, the length of the periods 

 being not perhaps exactly proportioned to the thick- 



