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 
will 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 
