42, THE DEPTHS OF THE SEA. [cHAP. I. 
maintains nearly the same conditions of temperature 
through many degrees of latitude; and when as it 
passes southwards it does become gradually affected 
by increasing warmth, it may be supposed merely to 
sink a few fathoms deeper, carrying its conditions and 
its fauna along with it. For example, animal forms 
which abound in the celtic province at 25 fathoms 
with a mean temperature of 10° C. may be expected 
in greatest number in perhaps 40 or 50 fathoms, 
with the same temperature, in the lusitanian province. 
Such a zone may thus be continuous for a great dis- 
tance, while the surface climate has been altering 
greatly, and the migrations of littoral forms have 
been again and again interrupted. But the deeper 
zone also sometimes meets with a ‘ natural barrier,’ 
as at the line of junction between the warm and cold 
areas already mentioned; which causes a curious 
sifting out of those species which are intolerant of a 
change of temperature. ‘hus the fauna of the tem- 
perate northward flow of water off the west coast of 
Scotland is materially different from that of the cold 
indraught along the east coast. 
If there be this overlapping bétween the lusitanian 
and celtic provinces, the same relation may be antici- 
pated between our own and the boreal province; and 
it is well known that this is the case, for the great 
majority of the mollusca which have been dredged by 
McAndrew, Barlee, and especially by Gwyn Jeffreys, 
from depths below 50 fathoms, are identical with those 
found in shallower water on the Scandinavian coast. 
Our recent work, while it has brought out more fully 
the overlapping, has gone much farther towards the 
indication of a general law. 
