Ch. V.] THE TERTIARY EPOCH. 57 



the irregular manner in which geological memorials are pre- 

 served, as already explained. We have little doubt that it 

 \vill be necessary hereafter to intercalate other periods, and 

 that many of the deposits, now referred to a single era,, will be 

 found to have been formed at very distinct periods of time, so 

 that, notwithstanding our separation of tertiary strata into four 

 groups, we shall continue to use the term contemporaneous 

 with a great deal of latitude. 



We throw out these hints, because we are apprehensive lest 

 zoological periods in Geology, like artificial divisions in other 

 branches of Natural History, should acquire too much impor- 

 tance, from being supposed to be founded on some great inter- 

 ruptions in the regular series of events in the organic world, 

 whereas, like the genera and orders in zoology and botany, 

 we ought to regard them as invented for the convenience of 

 systematic arrangement, always expecting to discover interme- 

 diate gradations between the boundary lines that we have first 

 drawn. 



In Natural History we select a certain species as a generic 

 type, and then arrange all its congeners in a series, according 

 to the degrees of their deviation from that type, or accord- 

 ing as they approach to the characters of the genus which pre- 

 cedes or follows. In like manner, we may select certain Geo- 

 logical formations as typical of particular epochs ; and having 

 accomplished this step, we may then arrange the groups referred 

 to the same period in chronological order, according as they 

 deviate in their organic contents from the normal groups, or 

 according as they approximate to the type of an antecedent or 

 subsequent epoch. 



If intermediate formations shall hereafter be found between 

 the Eocene and Miocene, and between those of the last period 

 and the Pliocene, we may still find an appropriate place for all, 

 by forming subdivisions on the same principle as that which 

 has determined us to separate the lower from the upper Plio- 

 cene groups. Thus, for example, we might have three divisions 

 of the Eocene epoch, the older, middle, and newer ; and 



