The Pein-ciples of Palaeontology. 209 



Quaternary are identical with, or at least very similar to forms 

 actually existing. Of these forms, some are now localized in 

 the northern seas, which they already inhabited at that epoch, 

 others have persisted on the same coasts ; lastly again, others 

 have migrated southward. A most interesting point for con- 

 sideration is tne order of succession of these forms. At the 

 beginning of the Pliocene epoch in the Coralline crag, the species 

 belonging to the warm seas had already entirely disappeared ; 

 in their place are found the temperate forms {Terebratulina 

 caput-serjpentis, Yoluta Lamberti, Astarte O'malii). Gradually the 

 Arctic forms make their appearance with Trophon antiquum in 

 the red crag, Cyprina islandica in the Norwich crag, and at 

 the same time such forms as Cardium edule, Turritella commu- 

 nis, which have continued to exist in the same seas. 



The crag of Anvers, with forms of the present temperate seas 

 such as Chenopus Jyf5-J?($Z^caw^, Isocardia cor, Saxicava artica, 

 shows also speeies of cold seas, such as Lucina horealis. The 

 existence of cold currents coming from the north, and bringing 

 progressively arctic forms, can not, therefore, be questioned. 

 Still, notwithstanding the gradual cooling, the temperature of 

 the solid land was yet, at the epoch of the Forest-hed, quite as 

 warm as at present ; this is proved by the study of the flora 

 and terrestrial shells. 



There are found at a still higher horizon deposits of the glacial 

 epoch. The raised beds of Yorkshire, of Scotland, of the Galles 

 country, the bowlder-clay which covers a great part of Eussia 

 and northern Germany, contain marine fossils, some of which 

 are identical with the present forms of the same regions {Cardium 

 edule, Ostrea edvZis, Bicccinum undatum, Murex erinaceus)\ 

 others have a clearly arctic character, such as Leda rostrata, 

 Fusus carinatus, Yoldia arctica. 



The phenomenon of the recession of pre-existing forms toward 

 the south, and the arrival of new forms from the north, becomes 

 progressively accentuated during a great part of the Quaternary 

 period. It seems an almost evident conclusion from these facts, 

 that a sensible cooling of the climate and the existence of cur- 

 rents from the north, have brought down the arctic fauna into 

 temperate latitudes. Some naturalists, nevertheless, have thought 

 the facts justified a contrary opinion, namely, that the Quaternary 

 fauna was autochthonic, and by emigration toward the north 

 had produced the actual arctic fauna. This phenomenon, then, 

 would be in accord with that which would people the great 

 depths through colonies from the littoral fauna, and would 

 explain the presence of forms common to the boreal and abyssal 

 zones, such as Rhizocrinus, Brisinga and numerous Mollusks. 



But these ingenious views are not confirmed ; they are contra- 

 dicted by numerous facts. The cooling of the earth at the gla- 

 cial Quaternary epoch is a fact so general and so well proved 

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