272 CONTEMPORANEITY OF STRATA AND THE 



would be either exclusively confined to the Carboniferous Limestone 

 in general, or which, perhaps, might not be found out of the Carboni- 

 ferous 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, fishes, mammals, insects, &c., which enjoy, when grown 

 up, the power of active locomotion, can actually " migrate" in person, 

 supposing they find themselves placed under unfavourable conditions. 

 There are many animals, however, such as most shell-fish, corals, 

 sea-urchins, &c., which have, when adult, either no power of changing 

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

 though the individual has no means of removing its quarters to some 

 more favoured spot, there may be a "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 for almost any distance, irrespective 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 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 Carboni- 

 ferous Limestone, probably the greater number of them, would 

 ultimately be transferred to aifbther area. Here they would mingle 

 with the forms already inhabiting that area, perhaps more or less 

 ■completely supplanting these, perhaps merely succeeding in main- 

 taining a precarious existence. In the course of their migration 

 jalso, they would doubtless become more or less modified in their 



