284 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



more precise than those of Secondary times, and 

 a great deal more so than those of Primary ones. 



But if palaeogeography can enlighten us upon the 

 phenomena of migration, on the other hand the 

 well-proved facts of the displacement of marine 

 and a fortiori of terrestrial animals bring to the 

 reconstitution of ancient geography decisive argu- 

 ments and uncontrovertible proofs. Thus it is 

 that the presence in England of the Mastodon 

 arvernensis, of the Elephas meridionalis, and of the 

 Mammoth to mention Proboscidians only im- 

 plies the existence of an isthmus connecting Eng- 

 land and France during the whole of a geological 

 phase extending from the Pliocene to the end of 

 Quaternary times. We have even been able to note 

 the very recent separation of Corsica from the 

 Continent of Provence by the discovery in that 

 island of a Stag (Cervus Cazioti) belonging to an 

 extinct group peculiar to the extreme end of the 

 Pliocene epoch. 



The study of the phenomena of migration offers 

 more complex conditions as regards marine animals 

 than in the case of continental faunas. Naturally, 

 when seas of large expanse are in question, the 

 migration of beings is not impeded by any material 

 obstacle, and the geographical distribution of faunas 

 is affected chiefly by the conditions of temperature 

 and depth of the waters of the sea. Accordingly 

 the distribution of certain genera is often very 

 extended in our existing seas, and it seems to have 

 been more so in the Secondary, and still more so 

 in the Primary ones. This specially applies to 

 animals which inhabit the mid-ocean, whether on 



