224 



OUTLIERS. 



remnants of the northern are supposed to be iso- 

 lated beneath the newer and existing fauna. 



If such be the real meaning of these local assem- 

 blages of northern forms from depths about our 

 islands, there will necessarily occur areas of sea-bed 

 at more moderate depths, where residual portions 

 of the northern are associated with the present 

 fauna. One such has been noticed by Mr. Jeffreys. 

 The Turbot bank off the coast of Antrim, in twenty- 

 five fathoms water, gave twenty-one species of Tes- 

 tacea, "Arctic," "Boreal," "Celtic," " Lusitanian f 

 all there assembled together. 



Zoological "outliers," therefore, can only be 

 looked for where the existing marine fauna is a 

 compound one, — the result of the admixture of 

 forms from adjacent areas ; they imply changes of 

 conditions over the areas in which they occur, both 

 as regards temperature and depth ; and inasmuch 

 as there is a tendency to uniformity at great depths, 

 the differences between provinces being mostly 

 found in the sublittoral zone, it follows that, though 

 there may be outlying southern species in northern 

 provinces, yet there can only be distinct northern 

 assemblages of species beneath seas which, in the 

 progress of change, have become warmer. A very 



