DOCTRINE OF GEOLOGICAL CONTINUITY. 281 



Cretaceous period," in any other sense than one might say, we are 

 living in the Silurian period ; with this difference only, that the 

 Cretaceous period is much nearer to us in point of time than the 

 Silurian, and that we can thus trace a relationship between certain 

 living types and certain Cretaceous forms, such as we can not hope to 

 establish in the case of Silurian fossils. 



Lastly, it is to be observed that certain classes of animals are 

 always likely to prevail under certain favouring conditions, wholly 

 irrespective of any generic connexion between successive faunse thus 

 represented. Thus, the conditions present in the deep Atlantic are 

 such as favour the existence of niimerous Foraminifera, siliceous 

 sponges, Echinoderms, and Brachiopods. Similar conditions existed 

 in the seas in which the chalk was deposited, and we need not, there- 

 fore, be surprised that similar groups of organisms abounded in the 

 cretaceous ocean. Similarly, there are portions of the incalculably 

 older Carboniferous Limestone fairly comparable to the chalk in 

 texture (making allowance for the vast difference of age), and con- 

 taining forms of life, which may be regarded as representative of the 

 Cretaceous fauna, such as Foraininifera, smooth Terebratulce,, and other 

 Brachiopods, with Crinoids and sea-urchins. The conditions, however, 

 present in the deep Atlantic cannot be exactly similar to those of 

 the Cretaceous seas ; for the Cephalopoda of the chalk seem to have no 

 representatives in the abyssal mud of the Atlantic, whilst this class 

 was well represented in Carboniferous times ; so that there is, if 

 ■anything, a closer genetic connexion between the chalk and the 

 Carboniferous Limestone than between the chalk and the Atlantic 

 -" ooze." 



