CONTEMPORANEITY OF STRATA. I/ 



many animals, however, such as most shell-fish, corals, sea- 

 urchins, &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 be a "migra- 

 tion " of the species from an unsuitable to a suitable locality. 

 This is effected through the medium of iheyottng, 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 re- 

 moval 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 consider- 

 ing : When the conditions of life in the seas of the Carbonifer- 

 ous 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 Carboniferous Limestone would ultimately 

 be transferred to some other area. Here they would mingle 

 with the forms already inhabiting that area, perhaps more or 

 less completely supplanting these, perhaps merely succeed- 

 ing in maintaining a more or less precarious existence. In 

 either case, their remains would be preserved in the sedimen- 

 tary deposits of the new area. When, ages afterwards, we come 

 to examine the crust of the earth geologically, we should find 

 these identical and characteristic species of fossils in the rocks 

 of the two areas, and we should say " these rocks are contem- 

 poraneous." It is clear, however, that we should be wrong in 

 so saying. The rocks in question would belong to the same 

 geological period, but they would belong to different stages of 

 the same period, and they would not be strictly contemporan- 

 eous. For deposits of this nature, believed to hold this relation 

 to each other, the term of " homotaxeous" has been proposed, 

 in place of the term " contemporaneous." 



What has just been said about the Carboniferous Rocks 

 would apply with equal justice to all the great formations, and 

 to many of the smaller rock-groups all over the world. The 

 Silurian Rocks of Europe, North America, South America, 

 Australia, &c., contain very similar fossils, and are undoubtedly 

 " homotaxeous." Nothing, however, that we see at the present 

 B 



