490 LTE OOLIN ALES OFM GIAOLO GVA 
do not reach into polar waters, animals can easily pass around the 
ends. Thus the molluscan fauna of the Mediterranean does not 
differ appreciably from that of the English waters, because in 
the passage around the peninsula of Spain animals remain in 
temperate waters, and under nearly the same conditions. 
On the other hand, the Isthmus of Panama separates two 
faunas absolutely distinct “from each other; the same thing is 
true of the Isthmus of Suez. ; 
East and west land masses would therefore not be very 
effectual barriers since they would not be so likely to extend into 
polar waters or into great differences of temperature. 
Chmatic zones—That climatic zones are today partial barriers 
to migrants along the coast is shown by the difference in faunas © 
living in northern and in southern latitudes under the same con- 
ditions. From the very nature of the case we should expect 
that cold water species would be able to cross climatic zones 
more easily than those adapted to warm water, since, as Dr. W. 
H. Dall* has shown, a fall of a very few degrees below the 
temperature favorable to life is more destructive than a rise of 
many degrees. 
In the present we have no means of testing this, but facts 
brought to light by geology confirm it. The Jura of Central 
Europe and of the Argentine Republic in South America have 
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. Also the faunas of the upper- 
most Jura and the lowest Cretaceous of Russia, which must have 
been Arctic in character, have migrated southward along the 
West Coast, almost into tropical waters. On the other hand we 
know of no case where equatorial faunas have passed through 
Arctic regions and even passages from tropical into temperate 
regions must be exceedingly difficult. 
The genera Lytoceras and Phylloceras are common in the 
Neocomian, Lower Cretaceous beds of southern Europe; but 
although these waters were undoubtedly in direct connection 
* Bull. Mus. Comp. Zool, Vol. XII., No. 6, p. 880. 
