50 INTRODUCTION. 



modating 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 

 number of species which would pass from the Carboniferous Lime- 

 stone 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. 



There would, in the second place, be a certain number of 

 species which would be utterly unable to withstand the altered con- 

 ditions 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 of a single 

 particular locality. 



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 locomotion, can actually " migrate " in person, sup- 

 posing they find themselves placed under unfavourable conditions. 

 There are many animals, however, such as most Molluscs, Corals, 

 Polyzoans, &c, which have, when adult, either no power of chang- 

 ing their place, or at best a very limited one. Still in these cases 

 even, though the individual has no means of removing his quarters 

 to some more favoured spot, there may bea" migration " of the 

 species from an unsuitable to a suitable locality. This is effected 

 through the medium of the young, which have the power of choosing 

 where they will settle, and are endowed with vigorous powers of 

 locomotion. If, for example, a bed of oysters should become placed 

 under conditions unsuitable for the development of these Molluscs, 

 it is clear that the old oysters cannot change their location. The 

 young oysters, however, swim about freely; and these will move 

 away from the original bed till they find a place which will suit them. 

 By a repetition of this process there may be in course of time a 

 removal or " migration " of a species to almost any distance, irre- 

 spective of the fact that the adult is permanently rooted. 



To return, then, to the case which we have been considering : 

 When the conditions of life in the seas of the Carboniferous Lime- 

 stone became unfavourable for the further existence of their fauna, 



