LES REVOLUTIONS DU GLOBE 15 



of continents once dry ; (3) other revolutions 

 resulting in the upheaval of mountain chains 

 have again cast back the waters and allowed, on 

 the foundation of the dried bottom of the sea, 

 the constitution of continental soils favourable to 

 the expansion of new terrestrial faunas ; (4) these 

 new faunas are not created on the spot, but come 

 from distant regions, their migration from which has 

 become possible owing to temporary bridges be- 

 tween continents. 



Without doubt geologists and palaeontologists 

 could not accept the whole of these propositions 

 without important reservations. It would seem 

 difficult for us at the present day, to admit for 

 instance the rapid destruction of terrestrial animals 

 by an incursion of the sea. The advances of the 

 sea appear to us rather as phenomena so rela- 

 tively slow and localized that it must always 

 have been possible for the inhabitants of districts 

 threatened by the sea to escape and to carry on in 

 safer quarters the peaceful continuity of their 

 evolution. 



No doubt also the influence which the upheavals of 

 mountain chains may have had on the retreats and 

 incursions of the sea may be questioned, and is much 

 disputed even nowadays. But none can deny the 

 paramount importance of migrations in the changes 

 of faunas, the repeated exchanges between the 

 populations of terrestrial animals passing from one 

 continent to another, the intermittent connections 

 established or destroyed by the phenomena of the 

 retreat or incursion of the sea, of the splitting 

 up, the subsidence, and the wrinkling of the earth's 



