40 INTRODUCTION. 



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 were 

 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. 

 Under any circumstances, some dispersion of the species of 

 the original Carboniferous area must have been going on by 

 the ordinary processes of migration from the commencement 

 of the Carboniferous period, but this dispersion must have 

 been greatly accelerated towards the close of the period of 

 the deposition of the Carboniferous Limestone. At this time 

 the conditions present in the original area must be supposed 

 to have become unsuitable for the further existence in that 

 area of the assemblage of animals which had been its inhabi- 

 tants, 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 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 effect of such a change 

 of conditions as we have supposed upon the animals inhabit- 

 ing the area ? a. Some of them would, doubtless, be suffi- 

 ciently hardy and accommodating to bear up under the new 

 state of things ; and these would persist 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 num- 

 ber 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 



