48 INTRODUCTION. 



contemporaneous, but that they succeeded each other in point of 

 time, though not necessarily by any very long interval geologically 

 speaking. The general ground for this presumption is the readily 

 intelligible one that such beds, if sufficiently far apart, must have 

 been deposited in different oceans, but that we cannot suppose that 

 any given species could have been developed to begin with in more 

 than one ocean. When, therefore, we find identical species in strata 

 in two widely remote areas, we are forced to conclude that these 

 species must have appeared sooner in one area than in the other, 

 and that the one set of strata must be later than the other, if by no 

 more than the time required for the migration of these species from 

 their original area to the other. 



Most of the facts bearing upon this question may be elicited by a 

 consideration of such a well-known and widely extended group of 

 deposits as the marine division of the Carboniferous system, of which 

 the chief member is the Carboniferous Limestone. This group of 

 deposits is more or less extensively developed in regions as remote 

 from one another as Europe, Central Asia, China, Japan, Australia, 

 South America, and North America ; and it is characterised by an 

 assemblage of distinctive fossils, among which certain species of 

 Brachiopods are specially noteworthy. Not only are the Carboniferous 

 Brachiopods in these widely distant areas referable to the same genera 

 (Producta, Athyris, Streptorhynchus, &c), but identical species are in 

 some cases found to range over the greater part of the vast area 

 occupied by these deposits. 1 Now, if we believe that the Lower 

 Carboniferous rocks in all these widely distant regions were " con- 

 temporaneous," in the sense that they were deposited at precisely 

 the same time, we should be compelled to admit the existence during 

 Carboniferous time of an ocean embracing all these points, and, in 

 spite of its enormous extent, so uniform in temperature, depth, and 

 the other conditions of marine life, that organisms either the same, 

 or very nearly the same, inhabited it from end to end. We can, 

 however, point to no such uniformity of conditions and consequent 

 uniformity of life over any such vast area at the present day; and we 

 have, therefore, no right to assume that this is the true explanation 

 of the facts. Moreover, all that we know of the geographical distri- 

 bution of recent organisms would prove that these identical species 

 cannot have been produced simultaneously in all the areas where 

 their remains are now found, but that they must have been dispersed 



1 Among the commoner and more characteristic types of Brachiopods found in 

 the Lower Carboniferous rocks of Europe, Producta semireticulata, P. longispina, 

 P. aculeata, P. costata, and Streptorhynchus crenistria occur, along with other 

 European forms, in the Carboniferous rocks of China (Kayser). All these species, 

 with the exception of P. costata, have been identified in the Carboniferous deposits 

 of Australia and India ; and all of them occur, with various other European 

 types, in the Lower Carboniferous rocks of North America. 



