342 
GEOLOGICAL SUCCESSION. 
Chap. X. 
only certain classes of organic beings have been largely 
preserved in a fossil state; that the number both of 
specimens and of species, preserved in our museums, is 
absolutely as nothing compared with the incalculable 
number of generations which must have passed away 
even during a single formation; that, owing to sub¬ 
sidence being necessary for the accumulation of fossili- 
ferous deposits thick enough to resist future degradation, 
enormous intervals of time have elapsed between the 
successive formations; that there has probably been 
more extinction during the periods of subsidence, and 
more variation during the periods of elevation, and 
during the latter the record will have been least per¬ 
fectly kept; that each single formation has not been 
continuously deposited ; that the duration of each 
formation is, perhaps, short compared with the average 
duration of specific forms; that migration has played 
an important part in the first appearance of new forms 
in any one area and formation; that widely ranging 
species are those which have varied most, and have 
oftenest given rise to new species; and that varieties 
have at first often been local. All these causes taken 
conjointly, must have tended to make the geological 
record extremely imperfect, and will to a large extent 
explain why we do not find interminable varieties, con¬ 
necting together all the extinct and existing forms of 
life by the finest graduated steps. 
He who rejects these views on the nature of the 
geological record, will rightly reject my whole theory. 
For he may ask in vain where are the numberless 
transitional links which must formerly have connected 
the closely allied or representative species, found in 
the several stages of the same great formation. He 
may disbelieve in the enormous intervals of time which 
have elapsed between our consecutive formations; he 
