of the Arctic and the Antarctic Faunas, 315 



certainly, in some cases even to species. If wo firul the 

 I'jaily Tertiary fauna, whose composition was " universal," 

 developed as far as the latitude of Copenhap^en in our 

 liemisf)here, there can hardly be any objection to the assump- 

 tion that in the southern hemisphere it was developed to 

 similar latitudes, and in that case it must have embraced all 

 the localities which now make up the area of the so-called 

 subantarctic fauna. When, owin^ to the gradual cooling of 

 the climate in the course of the Tertiary period, the com- 

 ponents of the old fauna of tropical habit withdrew from the 

 higher latitudes, and those remaining in the old place formed 

 a zonally-disposed relict-fauna, according to their power 

 of resistance to low temperature, identical or similar forms of 

 course remained behind in the corresponding northern and 

 southern latitudes, and not similar genera only but similar 

 species. Both from our own and from the Australian Mid- 

 Tertiary we know a number of species which have persisted 

 to the present day. In the same way quite a considerable 

 number of species have remained unaltered on the east and 

 west coasts of Central America since the Miocene period ; 

 and there is nothing to prevent our assuming that, in the 

 higher northern and southern latitudes also, a number of 

 species may have remained unaltered from the Mid-Tertiary 

 till now, and this could take place as well in the north as in 

 the south, so that, at the present day, identical species occur 

 in the northern and southern latitudes. 



If the components of the Early Tertiary faunas of tropical 

 habit withdrew from our latitudes towards the end of the 

 Early Tertiary, this process of selection or separating-out 

 must have taken place in higher latitudes proportionately 

 earlier, in the true polar zone certainly in the Cretaceous 

 period, if not before it. Now, no one assumes that animal 

 species (here I exclude the Protozoa) have remained un- 

 changed from the earliest Cretaceous period, or farther back, 

 until the present day. And if certain species actually occur 

 in higher southern latitudes which are also known from the 

 Arctic fauna, it is simplest to assume that these animals did 

 not remain behind in the j)olar zone in Mesozoic times, but 

 that they remained in the cooler temperate region in Tertiary 

 times, and thence extended their distribution towards the 

 pole. 



It may here be mentioned that it is not necessary to 

 picture the corresponding stages of the separation of the 

 faunas as quite simultaneous in both hemispheres; the result 

 is the same though corresponding phases in north and south 

 may not have taken place at exactly the same geological time. 



