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 sift- 

 ing out of those species which are intolerant of a 

 change of temperature. Thus 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 between 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, i 

 Our recent work, while it has brought out more fully j 

 the overlapping, has gone much farther towards the 

 indication of a general law. 



