SUBDIVISIONS IN GEOLOGICAL HISTORY. 403 



in fresh waters will differ in its fossils still more widely from that made 

 synchronously in salt waters ; a rock made in shallow waters from one made 

 at great depths ; a rock made in the tropics from one made in the temperate 

 zone or the arctic, provided the zones at the time of the making differed as 

 they do now in climate. Hence, a very considerable difference in the fossils 

 of rocks is consistent ivith their being contemporaneous in origin. 



2. As a consequence of the above facts, or the dependence of life on 

 food, temperature, and other physical conditions, migrations in species 

 or faunas will take place whenever there is a marked change in the waters; 

 it may be for a few miles or man3\ Barrande, first in 1852, pointed to 

 examples of such migrations in his " Colonies," as he styled them ; cases 

 of advanced occurrence locally of a fauna that afterwards disappeared, but 

 later became the prevailing fauna of a region, which he explained by migra- 

 tion, implying, as Geikie observes, that "particular species appeared with 

 the conditions favorable to their spread and disappeared when these ceased." 

 The case is the same when the fauna of a bed, which has apparently become 

 extinct, has recurrences in an overlying stratum whenever there is a recur- 

 rence of the kind of deposit. In and out the species go with the changing 

 conditions. Hence, as H. S. Williams has said, " the actual order of faunas 

 met with in a vertical section is not necessarily expressive of biological 

 sequence, but only of the sequence of the occupants of that particular area." 

 Sucii recurrences of species are likely to be met with in all regions where 

 fine shales, coarse shales, argillaceous sandstones, quartzose sandstones, with 

 or without limestones of varying purity, are in alternation. 



3. The difference in the time at which species or groups have begun to exist 

 in different regions. The several continents may not have been exactly 

 parallel, in all the steps of progress in the life of the globe. Certain families 

 may have commenced a little earlier in one than in another ; or again, one conti- 

 nental sea or region, over a continent, may have received some of its species 

 by migration from another, long after their first appearance. Here is a 

 source of doubt : what may be due, on one side, to special continental idiosyn- 

 crasies in condition or history, and, on the other, to migrational distribu- 

 tion, is always to be carefully considered. An example of the doubts and 

 difficulties which may be thus occasioned is afforded by the Cretaceous and 

 Tertiary formations of North America and Europe. Fossil plants of the 

 Rocky Mountain Cretaceous have been pronounced Tertiary by European 

 paleontologists who judged from comparisons with European Tertiary species ; 

 and yet the animal fossils of associated beds made it certain that they were 

 Cretaceous : and the query has thence arisen whether the European plants 

 may not be the successors of emigrants of Cretaceous American species 

 which, through this means, became characteristic in Europe of a post-Cre- 

 taceous period, or, whether the differences are not indigenous to the sepa- 

 rate continents. 



4. The difference in the time at which species or groups of species of differ- 

 ent regions have become extinct. In one region, changes may have caused 



