16 INTRODUCTION. 



large as the area now covered by the Pacific, or larger, and yet 

 it could not include all those localities in which strata of Car- 

 boniferous age with identical or representative fossils are 

 already kno>vn to exist. At the close of the deposition of 

 the Carboniferous Limestone in its original area, the condi- 

 tions there present must be supposed to have become unsuit- 

 able for the further existence in that area of the assemblage of 

 animals which had been its inhabitants, or, at any rate, 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 con- 

 nected 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 

 effect of such a change of conditions as we have supposed 

 upon the animals inhabiting the area? a. Some of them 

 would, doubtless, be sufficiently hardy and accommodating to 

 bear up under the new state of things ; and these would per- 

 sist into the ensuing period, 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 Car- 

 boniferous Limestone up into the Yoredale Series, the Mill- 

 stone 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. 



b. There 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 would be either exclusively confined to the 

 Carboniferous Limestone in general, or which, perhaps, might 

 not be found out of the Carboniferous Limestone of a single 

 region, or even a single particular locality. 



c. Lastly, some species would yield so far to the altered 

 conditions of the area that they would "migrate," and seek 

 elsewhere a more congenial home. This term is apt to convey 

 false impressions ; and it will be well here to consider what is 

 meant by the " migration " of species or groups of animals. It 

 is quite obvious that only animals like birds, mammals, insects, 

 &c., which enjoy when grown up the power of active locomo- 

 tion, can actually " migrate " in person, supposing they find 

 themselves placed under unfavourable conditions. There are 



