DOCTRINE OF GEOLOGICAL CONTINUITY. 271 



conditions over vast distances, sucL. as exists no -where at the present 

 day, and such as we have no right to assume ever existed. On the 

 contrary, the deposition of the Carboniferous Limestone must have 

 first taken place in one comparatively limited area — say in Europe — 

 ■where fitting conditions wei-e present both for the animals which 

 characterise it, and for the formation of beds of its peculiar mineral 

 and physical characters. How wide this area may have been, signifies 

 very little. It may have been as large as the area now covered by 

 the Pacific, or larger, and yet it could not include all those localities 

 in which strata of Carboniferous age with identical or representative 

 fossils are already known to exist. At the close of the deposition of 

 the Carboniferous Limestone in its original area, the conditions there 

 present must be supposed to have become unsuitable for the further 

 existence in that area of the assemblage of animals, which had been 

 its inhabitants, or, at any I'ate, for a great many of them. The 

 change from suitable to unsuitable conditions must, it is hardly 

 necessary to say, have been an extremely slow and gradual one ; and 

 would doubtless be connected with the progressive shallowing of the 

 sea, the diversion of old currents of heated water or the incoming of 

 new currents of cold water, or other physical changes tending to alter 

 the climatic conditions of the area. What, then, would be the result 

 of such a change of conditions as we have supposed upon the animals 

 inhabiting the area 1 



A. Some of them would, doubtless, be sufficiently hardy and 

 accommodating as to bear up under the new state of things; and 

 these would persist into the ensuing peiiod, without any perceptible 

 change, it might be, or more probably in the form of varieties or 

 species allied to the old ones. In this case, therefore, we should get 

 a certain number of species which would pass from the Carboniferous 

 Limestone up into the Yoredale series, the Millstone-grit or the Coal- 

 measures ; or, if we did not find any species exactly the same in all 

 these groups, we should still find in the later groups some forms which 

 would be varieties of those of the older, or which would be allied or 

 representative species. 



JB. Theie would, in the second place, be a certain number of species 

 which would be utterly unable to withstand the altered conditions of 

 the area; and these would gradually die out and become wholly 

 extinct. We should thus get a certain number of fossils, which 



