292 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



The species of these two faunas are only separated 

 by shades, at times almost imperceptible, but 

 remaining constant to a practised eye. Must it 

 necessarily be deduced from this relationship that 

 they are gradual mutations of the same type ? From 

 the absence of transitional forms, Fontannes does 

 not think so, but finds it quite reasonable to sup- 

 pose that the regular series of these mutations must 

 have occurred farther north, in a region unpene- 

 trated by the Pliocene sea and where the two Con- 

 tinental phases, Miocene and Pliocene, fuse into one 

 great epoch. I have succeeded in showing, with 

 Delafond, that these conditions are absolutely 

 realized in the small basin of the Bresse, which seems 

 to have had to play the part of a restocking centre 

 for the water courses and Pliocene lakes of the 

 southern part of the basin of the Rhone. 



Without there being any need to dwell at greater 

 length on these facts, we see what a paramount 

 part the phenomena of migration play in those 

 changes of faunas, sometimes so complete and 

 apparently so inexplicable, which we notice in the 

 various superposed stages of the marine formations. 

 Among the principal conditions which have in- 

 fluenced or determined these migrations, we have 

 been able to perceive : the direction of sea currents, 

 the perpetual oscillations of foreshores, and, lastly, 

 great marine incursions which carry with them the 

 inhabitants of distant seas, snatched, so to speak, 

 from their country of origin, to be transplanted, 

 like settlers, in points of the globe where their 

 ancestors did not exist. 



