THE MIGRATIONS OF MARINE ANIMALS 287 



South as far as the Equatorial region by following 

 the deep and cold double current which follows the 

 coast lines of Europe and of North America. From 

 these cold waters, becoming deeper and deeper as 

 they proceed towards the South, there has resulted a 

 curious adaptation of these types, of littoral habitat 

 in their birth place, to a more and more pronounced 

 deep-sea life as they approach the Equator. It is, 

 possibly, to the simple introduction of a deep and 

 cold current rather than to a general refrigeration 

 of our seas that should be attributed also the intro- 

 duction into the Mediterranean basin, towards the 

 end of the Pliocene era, of certain species of Arctic 

 sea shells, such as Trichotropis borealis, Astarte 

 borealis, and Trophon antiquum, which characterize 

 the Sicilian deposits round Palermo and a few 

 other points in the Mediterranean. 



The changes in the depth of waters, produced by 

 the positive or negative, though slight, oscillations 

 of the shore-lines, determine on their side the 

 emigration of whole faunas, the more so that to 

 the changes of a bathymetrical kind are added 

 parallel modifications in the nature of the sediments. 

 This is the phenomenon to which Van den Broeck 

 has given the picturesque name of Migration of 

 the Environment. Let a region of sandy beaches 

 be deepened by a flooding of the foreshore, and 

 slimy deposits will be seen to superpose themselves 

 on the sandy bottoms of the preceding period ; 

 and this change will suffice to determine both the 

 retreat, or even the local extinction of the early 

 inhabitants, and the introduction of other species 

 or genera which affect slimy soils, If ? on the con- 



