46 PRINCIPLES OF PALEONTOLOGY. 



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 is a 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 are actually contempo- 

 raneous, in the strict sense of the term, do not contain the same 

 fossils, if 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 



