CONTEMPORANEITY AND HOMOTAXIS. 5 I 



some species would migrate to a more congenial area. In this way 

 a greater or less number of the species characteristic of the Carbon- 

 iferous 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, or perhaps merely 

 succeeding in maintaining a more or less precarious existence. In 

 either case, their remains would be preserved in the sedimentary 

 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 contemporaneous." 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 contemporaneous. For deposits of this nature, be- 

 lieved to hold this relation to each other, the term of " homotaxial " 

 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 Ordovician and 

 Silurian rocks of Europe, North America, South America, Australia, 

 fee, contain very similar fossils, and are undoubtedly "homotaxial." 

 Nothing, however, that we see at the present day can justify us in 

 believing that these widely separated deposits are strictly " contem- 

 poraneous," in the sense that they were deposited at exactly the same 

 period of time. We should have to believe, if this conclusion is to 

 be justified, that in Ordovician and Silurian times the ocean spread 

 over a much larger area of the earth's surface than it does now, and 

 that its temperature and depth were unnaturally uniform"; and there 

 are, perhaps, some who would accept this view. What has been 

 said about the Ordovician and Silurian rocks as a whole, applies 

 with still greater force to certain of the minor subdivisions of the 

 same, which contain many of the same specific forms in parts of 

 the globe very widely removed from one another. It is the very 

 identity of the fossils, however, which proves that the beds in ques- 

 tion, from their geographical position, cannot have been deposited 

 at exactly the same time, though they doubtless belong to the same 

 period, and may even be said to be related to one another, so far 

 as their identical fossils are concerned, by lineal descent. Similar 

 remarks might be made about the Devonian, Permian, Triassic, Ju- 

 rassic, Cretaceous, and other formations ; but it is not necessary 

 further to multiply examples. 



If we consider the present state of things upon the globe, we 

 shall be further convinced of the justice of these views, which were 

 first prominently brought forward in Britain by Professor Huxley. 



