42 INTRODUCTION. 



sidering : When the conditions of life in the seas of the Car- 

 boniferous Limestone became unfavourable for the further 

 existence of their fauna, some species would migrate to a 

 more congenial area. In this way a greater or less number 

 of the species characteristic of the Carboniferous Limestone 

 would ultimately be transferred to some other area. Here 

 they would mingle with the forms already inhabiting that 

 area, perhaps more or less completely supplanting these, 

 perhaps merely succeeding in maintaining a more or less 

 precarious existence. In either case, their remains would 

 be preserved in the sedimentary deposits of the new area. 

 When, ages afterwards, we come to examine the crust of 

 the earth geologically, we should find these identical and 

 characteristic species of fossils in the rocks of the two 

 areas, and we should say "these rocks are contemporan- 

 eous." It is clear, however, that we should be wrong in 

 so saying. The rocks in question would belong to the same 

 geological period, but they would belong to different stages 

 of the same period, and they would not be strictly con- 

 temporaneous. For deposits of this nature, believed to hold 

 this relation to each other, the term of " homotaxeous " has 

 been proposed, in place of the term " contemporaneous." 



What has just been said about the Carboniferous rocks 

 would apply with equal justice to all the great formations, 

 and to many of the smaller rock-groups all over the world. 

 The Silurian rocks of Europe, North America, South 

 America, Australia, &c., contain very similar fossils, and are 

 undoubtedly " homotaxeous." Nothing, however, that we 

 see at the present day can justify us in believing that these 

 widely separated deposits are strictly "contemporaneous," 

 in the sense that they were deposited at exactly the same 

 period of time. We should have to believe, if this conclu- 

 sion is to be justified, that in Silurian times the ocean 

 spread over a much larger area of the earth's surface 

 than it does now, and that its temperature and depth were 

 unnaturally uniform ; and there are, perhaps, some who 

 would accept this view. What has been said about the 

 Silurian rocks as a whole applies with still greater force 

 to certain of the minor subdivisions of the same, which 



