688 JAMES PERRIN SMITH 



That climatic zones alone are to-day partial barriers to 

 migrants along the coast is shown by the difference in faunas 

 living in northern and in southern latitudes on north-south 

 shores. We would expect cold water species to be able to cross 

 climatic zones more easily than those adapted to warm water. 

 But we know of no cases where equatorial faunas have passed 

 through arctic regions, and even passages from tropical into tem- 

 perate waters must have been exceedingly difficult, for a fall of 

 a few degrees below the temperature favorable to life must be 

 a great deal more destructive than a rise of many degrees. At 

 present we have no means of testing this statement, but facts 

 brought to light by geology confirm it. The Jura of western 

 Europe and of the Argentine Republic have practically the same 

 fauna, which, in reaching one of these regions from the other, 

 must have passed from temperate waters through tropical, and 

 into temperate seas again. The genera Lytoceras and Phylloceras 

 are common in the Neocomian beds of southern Europe ; but 

 although these waters were undoubtedly connected with those of 

 northern Europe, those genera are lacking in the latter region. 

 Also in the lower part of the Californian Knoxville beds, the 

 above mentioned genera are unknown, and come in only higher 

 up where the first members of the tropical Indian fauna began to 

 appear. 



By far the greater part of marine animals live near the shore 

 and are unable to exist under other conditions. To these an 

 abyssal sea is as impassable a barrier as a continent. The marine 

 faunas of the southern ends of Africa. South America, and Aus- 

 tralia are in approximately the same climatic conditions, but 

 although they are connected by open seas, they are as different 

 as if they were in totally disconnected basins. But an east-west 

 sea affords good opportunity for passage from one side to the 

 other by slow passage along the margin. The present fauna of 

 the Mediterranean is good evidence of this, the animals of the 

 European shores not differing appreciably from those of the 

 African. The Mesozoic faunas of the ancient Central-Mediter- 

 ranean sea owe their great distribution to this fact, for nearly the 



