46 PRINCIPLES OF PALEONTOLOGY. 



of the word " contemporaneous, " and we must bear in mind 

 the enormously-prolonged periods of time with which the 

 geologist has to deal. When we say that two groups of strata 

 in different regions are " contemporaneous, " we simply mean 

 that they were formed during the same geological period, and 

 perhaps at different stages of that period, and we do not mean to 

 imply that they were formed at precisely the same instant of time. 

 A moment's consideration will show us that it is only in the 

 former sense that we can properly speak of strata being " con- 

 temporaneous ; " and that, in points of fact, beds containing 

 the same fossils, if occurring in widely distant areas, can hardly 

 be " contemporaneous " in any literal sense ; but that the very 

 identity of their fossils is proof that they were deposited one 

 after the other. If we find strata containing identical fossils 

 within the limits of a single geographical region say in Europe 

 then there is a reasonable probability that these beds are 

 strictly contemporaneous, in the sense that they were deposited 

 at the same time. There is a reasonable probability of this, 

 because there is no improbability involved in the idea of an 

 ocean occupying the whole area of Europe, and peopled 

 throughout by many of the same species of marine animals. At 

 the present day, for example, many identical species of animals 

 are found living on the .western coasts of Britain and the eastern 

 coasts of North America, and beds now in course of deposition 

 off the shores of Ireland and the seaboard of the state of 

 New York would necessarily contain many of the same fossils. 

 Such beds would be both literally and geologically contempora- 

 neous ; but the case is different if the distance between the areas 

 where the strata occur be greatly increased. We find, for ex- 

 ample, beds containing identical fossils (the Quebec or Skiddaw 

 beds) in Sweden, in the north of England, in Canada, and in 

 Australia. Now, if all these beds were contemporaneous, in the 

 literal sense of the term, we should have to suppose that the 

 ocean at one time extended uninterruptedly between all these 

 points, and was peopled throughout the vast area thus indicated 

 by many of the same animals. Nothing, however, that we see at 

 the present day would justify us in imagining an ocean of such 

 enormous extent, and at the same time so uniform in its depth, 

 temperature, and other conditions of marine life, as to allow 

 the same animals to flourish in it from end to end ; and the 

 example chosen is only one of a long and ever-recurring series. 

 It is therefore much more reasonable to explain this, and all 

 similar cases, as owing to the migration of the fauna, in whole 



