46 PRINCIPLES OF PALAZONTOLOGY. 
will entomb the remains of the animals as fossils. After this 
has lasted for a certain length of time, the European area may 
undergo elevation, or may become otherwise unsuitable for the 
perpetuation of its fauna; the result of which would be that 
some or all of the marine animals of the area would migrate to 
some more suitable region. Sediments would then be accumu- 
lated in the new area to which they had betaken themselves, 
and they would then appear, for the second time, as fossils in 
a set of beds widely separated from Europe. The second set 
of beds would, however, obviously not be strictly or literally 
contemporaneous with the first, but would be separated from 
them by the period of time required for the migration of the 
animals from the one area into the other. It is only in a wide 
and comprehensive sense that such strata can be said to be 
contemporaneous. 
It is impossible to enter further into this subject here; but 
it may be taken as certain that beds in widely remote geogra- 
phical areas can only come to contain the same fossils by 
reason of a migration having taken place of the animals of 
the one area to the other. That such migrations can and do 
take place is quite certain, and this isa much more reasonable 
explanation of the observed facts than the hypothesis that in 
former periods the conditions of life were much more uniform 
than they are at present, and that, consequently, the same 
organisms were able to range over the entire globe at the same 
time. It need only be added, that taking the evidence of the 
present as explaining the phenomena of the past—the only 
safe method of reasoning in geological matters—we have 
abundant proof that deposits which ave actually contempo- 
raneous, in the strict sense of the term, do not contain the same 
Jossils, of far removed from one another in point of distance. 
Thus, deposits of various kinds are now in process of forma- 
tion in our existing seas, as, for example, in the Arctic Ocean, 
the Atlantic, and the Pacific, and many of these deposits are 
known to us by actual examination and observation with the 
sounding-lead and dredge. But it is hardly necessary to add 
that the animal remains contained in these deposits—the 
fossils of some future period—instead of being identical, are 
widely different from one another in their characters. 
We have seen, then, that the entire stratified series is capable 
of subdivision into a number of definite rock-groups or ‘‘forma- 
tions,” each possessing a peculiar and characteristic assem- 
blage of fossils, representing the “life” of the “period” in 
which the formation was deposited. We have still to inquire 
shortly how it came to pass that two successive formations 
