ZOOLOGICAL TRANSITIONS THROUGHOUT THE OLDER ROCKS. 



585 



epochs, is not manacled by present conditions, and if the phenomena to which he appeals 

 convince him, that the former changes of the planet were of surpassing grandeur, he 

 may also conclude, that some general cause must explain the distribution of the same 

 species over distant regions. If the existence of formations, so nearly universal as respects 

 the surface of the globe, be admitted, it would seem to be a fair inference, that however 

 we may explain it, there must have then prevailed a generally equable temperature. 



But let us pass from these general views to the immediate objects of this work. 



The following pages and accompanying plates are intended to develop the amount 

 of zoological mutation throughout a large portion of these ancient fossiliferous strata } 

 and the inference we obtain is, that most of the groups which graduate into each other 

 by lithological and geological characters, exhibit also a true zoological transition. Thus, 

 in reviewing this ancient succession in ascending order, we perceive a certain number 

 of species common to the Upper Formations of the Cambrian System and the Lower 

 Silurian Rocks ; and that while some of the Lower Silurian animals lived on to those 

 days when the Upper Silurian beds were deposited, scarcely a single species which existed 

 when the Silurian sera commenced, can be detected in the strata which mark its close. 



Again, in proceeding upwards from the Silurian period into that of the Old Red Sand* 

 stone, we find in its lowest member, a partial intermixture of a few fossils, typical of the 

 Upper Ludlow Rock or the last-formed beds of the Silurian System, associated with 

 some new species ; and ascending higher amid these red deposits, we meet with fishes 

 of entirely distinct genera and species, as unlike those of the Silurian sera as they are 

 to those of the overlying Carboniferous System. In the upper division, however, of 

 the Old Red System (though of enormous thickness) scarcely the trace of fossil animals 

 has yet been detected, and hence we are unprovided with zoological links to connect 

 the whole series, though I have no doubt that such proofs will be hereafter discovered, 

 and that we shall then see in them, as perfect evidences of a transition between the Old 

 Red and Carboniferous rocks, as we now trace from the Cambrian, through the Silurian, 

 into the Old Red System. 



When, therefore, it is asserted, that the fossils of the Carboniferous sera are dissimilar 

 from those of the Silurian, the reader must bear in mind, that the strata so broadly 

 distinguished, are separated by accumulations of enormous thickness, and that the vast 

 time occupied in their deposit, accounts satisfactorily for an almost entire change in 

 the forms of animal life. 



The same order and change of species is apparent throughout the overlying for- 

 mations. Thus, although there is a slight community of character between the upper 

 members of the Carboniferous rocks and the lower strata of the New Red System, 

 particularly in the plants 1 j and although certain mollusca of the Magnesian Limestone 



1 I have adverted slightly in this volume, to the vegetables which may he found in the Silurian System ; for 

 although I am persuaded of the truth of the induction of M. Adolphe Brongniart and other fossil botanists, 

 that each geological period, from the tertiary down to the carboniferous deposits, is marked by a distinct Flora, 



