8 
Classification of Hocks 
place, although the species, whether of existing or extinct 
contemporaneous beings, may be specifically distinct in 
different countries, there are mostly genera common to 
both, especially in the lower orders of animals, and always 
a peculiar character that enables us to class them in their 
proper places. Secondly, where the same formation at 
two distant localities contains different organic remains, 
research at intermediate points will enable us to detect 
the transition, which must almost necessarily be a gradual 
one. And, thirdly, it appears that even the specific dif¬ 
ferences between organic beings at distant localities were 
not so great in the more ancient periods of the world’s 
history, as they were during the tertiary epochs, or are at 
the present time. 
The last and most unerring proof of the place of any 
rock or formation in the series is gained by working out 
its order of superposition by actual sections and field 
operations. The best zoologists, botanists, or compara¬ 
tive anatomists, may err in their decision on the species 
of a fossil animal or plant; but patient labour and careful 
and widely extended observation in actually tracing their 
course and position can hardly fail to ascertain the series 
of rocks of which any country is composed, and place 
them in their due order and arrangement. In this order, 
when founded on sections sufficiently clear and extensive, 
there can be no mistake. When, moreover, the order of 
superposition of the rocks and formations in any one line 
of country is made out, and another tract is to be com¬ 
pared with it, the only certain method is to connect the 
two by positively traversing the whole intermediate space, 
and never losing hold of the clue obtained at one end, 
until it fairly conducts us to the other. It is plain, how¬ 
ever, that this method is rigorously confined to those 
countries only which are connected by dry land. When 
countries such as Europe and Australia come to be com- 
