of the Arctic and the Antarctic Faunas. B15 
certainly, in some cases even to species. If we find the 
Karly ‘Tertiary fauna, whose composition was “ universal,” 
developed as far as the latitude of Copenhagen in our 
hemisphere, 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, owing 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 Karly 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 trom 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 polar 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. 
