CONTEMPORANEITY OF STRATA. 41 



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 Lime- 

 stone 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 con- 

 vey 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, supposing they find themselves placed under un- 

 favourable 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 his 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 en- 

 dowed 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, irrespective of the fact 

 that the adult is permanently rooted. 



To return, then, to the case which we have been con- 



