AvucustT 8, 1912] 
or shutting of these gates, and of others leading to 
more southern seas, that the geologist ascribes much 
of the successive changes of climate and of fauna 
during Tertiary times. 
The topography of the North Sea, as well as of 
our land, bears its fragmentary records of these old 
times. The Dogger Bank is perhaps but a great 
moraine, and over it (when the great ice-cap had 
passed away) roamed the rhinoceros, the reindeer, 
and the mammoth. The deep groove off Norway was 
probably in part a channel whereby the river system 
of eastern Europe ran seaward, in part an eddy, where 
the Scandinavian glaciers gripped and scooped their 
hardest, and, first of all, probably a great crumple in 
the earth’s crust. In the Moray Firth a deep channel, 
more than a hundred fathoms deep, exists; it is the 
course by which an older and greater Spey ran 
tributary into an older and greater Rhine. 
Apart from the great tidal waves that roll in twice 
a day from the ocean round by our northern and 
southern gates, the great dominating movement of 
our seas lies in the Atlantic current, or system of 
currents, that we commonly call the Gulf Stream. 
The Gulf Stream itself is a river in the ocean (as 
Maury called it); but partly as a river, and in part 
as a great, wide, slow-drifting flood, the warm waters 
of the bosom of the Atlantic creep ever northward and 
eastward to bathe our shores, and to soften the climate 
of sea and land in northern Norway and distant Spitz- 
bergen. A little branch of the current enters in by 
the southern gate, a somewhat greater eddies round 
the north of Scotland, and under these two impulses 
(aided by local differences in the density of the waters 
of the North Sea basin) a circling current flows down 
our eastern coasts, across to Denmark, and in part 
out again along the Norwegian shore. The direct 
influence of this system of currents on the life of 
fishes is immense, for by its means their floating 
eggs and young are dispersed and disseminated broad- 
cast. In the south those of the plaice and sole are 
carried over to their nursery grounds on the flat 
Danish shore; and in like manner the eggs and fry 
of the cod are drifted from the western coasts round 
the north of Scotland into the North Sea, and in 
part out again to the Sea of Norway. 
Simply and clearly we may see by our chart the 
distribution of temperature in the North Atlantic, 
due, on one hand, to the Gulf Stream current, and 
on the other to the opposing currents from the pole, 
that bend westward in their southerly course and 
cool the Newfoundland Banks and the shores of the 
Eastern States, while a minor offshoot from the direc- 
tion of Iceland, submerged beneath the warm Atlantic 
waters, approaches or invades our own seas. We see, | 
in passing, the close-pressed isotherms on the New- 
foundland Banks, where the two waters meet, and we 
may note, by the way, that it seems to be a fact that 
fish tend to accumulate just at such meeting places 
of different waters. But looking broadly at our own 
temperature phenomena the most striking points are: 
our western coast bathed by the warm current, the 
eastern remote from its influence; again, the rapid 
change of temperature from the favoured regions of 
southern Ireland and south-western England as we | 
go farther north; and, lastly, the uniformity of tem- 
perature over the wide region that sweeps round | 
from the North Sea by wav of Iceland all round the 
North Atlantic to Newfoundland again. 
The difference of temperature between our western 
or southern coasts and the eastern is in close relation 
with the great contrast between the fish of the two | 
regions. Broadly speaking, to the former belong 
southern fishes, while fishes the home and distribution 
of which are in the north characterise the latter. 
There cannot be a more striking contrast than that | 
NO. 2232, voL. 89] 
NATURE 
595 
between one of our fish markets and a market of 
Lisbon, Genoa, or Marseilles. The cod and the 
haddock, and nearly all their allies (save the hake) 
are absent from the latter; flat fish are few, and the 
great order of the spiny-finned fishes, the bream and 
the sea perch, the mullet, the gurnard, and a multi- 
tude of others, mostly alien to our markets and 
strange to our eyes, form the staple commodity. A 
difference, similar in kind though less in degree, 
exists between our western fisheries and those of the 
North Sea. The pilchard, the chief Clupeoid of the 
Atlantic coasts, finds its appropriate temperature on 
the Cornish coast, and rarely penetrates the colder 
waters to the east. The hake, which takes the place 
of the cod along the Atlantic seaboard, comes round 
indeed into the North Sea with the Gulf Stream 
eddy, but in meagre quantities. The bream, which 
both fresh and salted is an important food of the 
poor on the west of Ireland, is not in the North Sea 
an article of commerce. The trawlers that seek the 
coasts of the Spanish Peninsula and of Morocco find 
in these warm waters a fishery totally unlike that of 
the North Sea; while, on the other hand, our tem- 
perature curves make it plain and easy for us to 
understand how the North Sea has common attributes 
with regions so far off as the White Sea itself, with 
Iceland and Newfoundland and the Eastern States, 
and how our staple fishes, such as the cod, the had- 
dock, the plaice and halibut, and the herring itself, 
find their extensive distribution in all these remote, 
but not dissimilar seas. 
Lastly, ere we leave this matter of temperature, let 
me point out to you that the ocean not only acts in 
this part of the world as a warming influence, but also 
here and everywhere has a great steadying influence 
upon the temperatures. In another chart I show, 
not the mean temperatures, but the range of tem- 
perature, the difference between the summer heat of 
the sea and its winter cold. A little way west of 
Ireland the annual range of temperature is but 4°, 
and in Shetland it is but 6°; but the further we go 
into the narrow seas the more violent is the seasonal 
fluctuation, the greater is its range, until down in 
the German bight you have a range of at least 12° 
or 14° C., or 30° to 35° F. The water there is far 
colder in winter than in other parts of our sea. But 
there comes a great compensatory warmth in summer, 
which again has its influence in favouring this region 
as a nursery for young fish. 
The problem of salinity, the distribution of the 
amount of salt in the sea, is a laborious one to in- 
vestigate, but, so far as the North Sea goes, its 
main results are easy to understand. Some of you 
will see at a glance from this chart of Isohalines, 
how beautifully simple the arrangement is, and how 
perfectly it is in accord with the distribution of the 
three gateways of the sea, the two inlets of salt water, 
and the great Baltic source of fresh. But the further 
study of the salinity of the North Sea is very com- 
plicated indeed, for the mean condition which my 
chart represents is subject to change, and the changes 
are partly regular or periodic, and partly irregular 
and obscure. There is a constant battle, as it were, 
between the quantities of fresh water, on one hand, 
that the Baltic sends in, and the rivers bring down 
(for the former source especially tends to be dried 
up when the inland sea is frozen in winter time), and 
the varying supply of salt water from the ocean, for 
even the great ocean currents have their annual pulse, 
their ebb and flow. In the summer time over great 
| part of the North Sea, water of low salinity spreads 
from the Baltic, and such changes as_ this 
have, we have every reason to believe, their close 
and intimate bearing on the migrations of the herring. 
Lastly, together with these physical phenomena of 
