DISTRIBUTION IN TIME. 155 



at the same period in the history of the earth, and belong 

 to the same geological epoch. 



This statement, however, can only be received with some 

 important qualifications. Beds containing the same specific 

 forms are often so widely removed from one another in 

 point of distance, and occur at so many different points of 

 the earth's surface, that it becomes inconceivable that they 

 are "contemporaneous" in the Hteral sense of this term. 

 Such a supposition would imply an ocean not only more 

 widely extended but presenting more uniform conditions 

 than any with which we are at the present day acquainted. 

 Besides, we know that strictly contemporaneous beds would 

 rarely contain exactly the same species of fossils. Thus, if 

 we could examine the bed of the Atlantic, we should un- 

 doubtedly find it occupied by a series of deposits which 

 would be "contemporaneous" in the strictest sense of the 

 term, but they would neither have the same mineral char- 

 acters, nor contain the same or even nearly-related fossils. 

 Some of the deposits, for example, would consist of chalky 

 beds, crowded with Foraminifera, Siliceous Sponges, Crinoids, 

 and Sea-urchins. Others would be composed of sand and 

 mud, and would contain the remains of Arctic shells. 

 Others, again, would have the characters of shore-deposits, 

 and would yield the remains of littoral animals. If this be 

 the case with a single ocean, such as the Atlantic, still more 

 is it the case when we consider all the oceans of the globe, 

 the deposits of which are nevertheless contemporaneous, in 

 the sense that they have been formed at the same time. 



Contemporaneous beds, then, if separated from one 

 another in point of distance, are by no means likely to 

 contain the same species of fossils. We are thus driven to 

 seek for another explanation of the fact that specifically 

 identical fossils are often found in formations very widely 

 removed from one another. The true explanation of this 

 fact is to be sought in the phenomenon of " migration." 

 If we imagine a given assemblage of animals to be inhabit- 



