ur)4 
THE VOYAGE OF H.M.S. CHALLENGER. 
intervening 
tropics, been taken into 
IN Okiths 
Greatek than, 
AND Less than. 
.•VOO Fathoms. 
southern zones of latitude, and not in the 
consideration. 
Let us now limit our attention to species taken in the southern zone of latitude in 
depths greater than 500 fathoms which may be considered as representing the 
<leep-sea fauna. To the south of the tropic of Capricorn the Challenger pro- 
Si'E. Its C.uTLRED cured 793 species from depths greater than 500 fathoms, all of which live on 
the bottom or close to it. 610 of these, or 76 '9 per cent., are limited to the 
southern zone of latitude, 48 species, or 6 per cent., occur in all the three 
zones, and 52 species, or 6 ’5 per cent., occur in the northern and southern zones 
and not in the intervening tropics. Nearly similar results are obtained by limiting 
our examination to the species captured deeper than 1000 fathoms, 1500 fathoms, and 
so on. If we now turn our attention to the animals captured in the shallower zones in 
the southern zone of latitude, it will be found that the species captured in depths less 
than 500 fathoms numbered 1875, and of these 102 species, or 5 ’4 per cent., occur in 
the northern zone, and not in the intervening tropics. Speaking generally, the per- 
centage of species occurring in all the three great zones of latitude is slightly higher 
in the deeper zones than in the shallower, but the percentage of species which occur in 
the northern and southern zones, and not in the intervening tropics, remains nearly 
the same in deep and in shallow water when the species given on pages 1446 to 1450 
are taken into consideration. From this examination again, it does not appear that 
there is any sufficient evidence for the belief in a universal deep-sea fauna of great 
antiquity. There are many indications that the migration into the deep sea has taken 
place continuously since Mesozoic times, when possibly cooling set in at the poles, and 
is even now going on. The indications that this migration is taking place principally 
from polar regions are more distinct than those of migration from any other quarter, 
although instances might be cited of groups now living in tropical regions about the 
mud-line which have apparently a much wider distribution in the deep sea. 
In all the shallow- water dredgings about the islands of the Great Southern Ocean, 
such a.s Kerguelen, Heard Island, the Falklands, and Crozets, numerous species of 
the Isopod genus St'rolifi were procured on muddy deposits. Species of this genus, 
modified for deep-.sea life, were likewise obtained in diminished numbers at great depths 
in high southern latitudes, and some large species were dredged in very deep water as 
far north as the equator, although the genus is not represented in the littoral region in 
these latitudes, and, with one doubtful exception, there do not seem to be any repre- 
sentatives of the genus north of the equator. This may be taken as an instance of how 
animals have spread from the mud-line to deeper zones and from the Antarctic area 
northward over the floor of the ocean, but it is altogether improbable that littoral 
.Species can, as has been suggested, jiass by way of the deep sea from the littoral zone of 
one polar region to that of the other. 
I M.-IRIBCTIO.N OF 
TIIF. GkSCH 
.'ifTWM.IS. 
