of the Arctic and the Antarctic Faunas. 317 
we cannot say whether it has migrated from the north or from 
the south; but asevery migration demands time, we can assume 
with some probability that those occurring in the temperate 
zone of the northern hemisphere have come from the north, and 
those in the southern from the south. But if it were the case 
that the time which a species requires to migrate over the 
whole deep sea from one polar zone to the other were trifling 
in comparison with the length of its sojourn in the deep sea, 
one could no longer say that an example found near Scotland 
came from the north, and one found near South Georgia came 
from the south. But this is not at all how matters stand. 
Murray has compiled exact statistics of distribution for the 
Kerguelen region; I myself have extended these for the 
whole earth, though they are still far from being complete. 
But one thing seems fairly well established, that practically 
all the unipolar surface and subsurface animals of the higher 
north and south, which descended into the deep sea, have 
penetrated to the borders of the tropics or into the tropical 
zone, but not beyond it into the opposite hemisphere. An 
example known to most zoologists is furnished by the genus 
Serolis, of which many species are developed in the notal 
surface-water, and a still greater number in the deep sea, yet 
its range, apparently, does not extend beyond the equator. 
It would seem, therefore, that the time which has elapsed 
since the present surface-water species of the higher north 
and south descended to the depths has not sufticed for a 
migration beyond the equator to the opposite hemisphere; 
the exceptions to this rule disappear almost wholly, if not 
wholly, on closer consideration, although for certain species 
of Sponges, Worms, and Bryozoa we must assume an age 
extending beyond the middle ‘Tertiary period—and this is in 
no way at variance with the facts. 
SUBSURFACE-F'AUNA. 
We have now to deal in a few words with the subsurface, in 
the same way as we have dealt with the deeper water. We 
know that in Early Tertiary times a universally homogeneous 
fauna extended over the tropics and the temperate zones. 
Thus the similar species of north and south had a continuous 
connexion through the tropical zone. ‘This continuity through 
the tropical zone was probably kept up in part through the 
deep water. As within the tropical zone at the present day, the 
fauna of northern habit is found exclusively in the subsurtace- 
water (we shall have to consider later the peculiar conditions 
of Western America), nothing is more obvious than that there 
Ann. & Mag. N. Hist. Ser. 7. Vol. vii. 22 
