INVERTEBRATE PALAEONTOLOGY 



emphasize the danger of forming opinions on such a 

 matter from the evidence provided within a small area. 



In Britain, the arbitrary divisions between the Lower 

 and Upper Palaeozoic (Proterozoic and Deuterozoic)> 

 Palaeozoic and Mesozoic, and Mesozoic and Cainozoic, 

 conform so closely to observed facts in Stratigraphy 

 and Palaeontology that they appear to coincide with 

 natural breaks. Not only were the Upper Silurian, 

 Permo-Carboniferous and Late Cretaceous periods 

 marked by serious earth-movement (involving important 

 physiographical changes), but they were followed by 

 episodes which, whether in sedimentation or denudation, 

 have left great lacunae in palaeontological evidence. 

 In consequence, the faunas separated by these gaps in 

 the record prove to be exceedingly different in character. 

 The fossils of the Magnesian Limestone seem to have 

 little connexion with those of the Rhaetic and Lias (the 

 next truly marine series accessible in this country). 

 Between the organisms of the Chalk sea and those of 

 the Eocene estuaries there is a contrast that cannot 

 wholly be ascribed to diverse physiographical conditions. 

 It is difficult to avoid the conception that some great 

 acceleration must have influenced evolution during the 

 barren intervals. Where positive and negative evidence 

 is involved, a kind of mental halation tends to magnify 

 the impression of the former at the expense of the 

 latter. 



Indication of the unsoundness of such reasoning can 

 be found, in purely palaeontological features, even in 

 this country. The scanty and ill-preserved fauna of the 

 Devonian contains a blend of types (particularly among 

 Coelenterates, Brachiopods and Trilobites) ^which have 

 as much in common with their Silurian predecessors 

 as with their Carboniferous descendants. On strati- 

 graphical grounds, the vast accumulation of Permo- 



