BREAKS IN THE GEOLOGICAL RECORD. 47 



or in part, from one marine area to another. Thus, we may 

 suppose an ocean to cover what is now the European area, and 

 to be peopled by certain species of animals. Beds of sediment 

 clay, sands, and limestones will be deposited over the sea- 

 bottom, and 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 con- 

 temporaneous. 



It is impossible to enter further into this subject here ; but it 

 may be taken as certain that beds in widely remote geographical 

 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 contemporaneous, 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 formation 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 obser- 

 vation with the sounding-lead and dredge. But it is hardly neces- 

 sary 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 serioe is 



