GENERAL INTRODUCTION TO THE ZOOLOGICAL RErORTS. 49 



northern and soutliern Polar, areas the isotherm of the mean temperature of 36° F. rises 

 towards the surface, and we find in the seas of Scandinavia and Kerguelen Island, at 

 moderate depths, a mixture of abyssal types, with the fauna of what Professor Edward 

 Forbes defined as the iiifra-median zone. 



III. Pressure and the absence of light appear to affect animals to a certain extent. 

 No difierences exist in the composition or the comparative salinity of the sea-water of 

 the abyssal region, or in the amount, or the proportion, or the nature of the gases dis- 

 solved in it, to afiect animal life. 



IV. The abyssal fauna is very special, and remarkably uniform throughout the 

 enormous area over which it extends. It is characterised by the abundance and variety 

 of certain conspicuous invertebrate groups, which are either unrepresented or play a very 

 subordinate part in the shallow-water faunae. 



V. In all probability the depressions in the crust of the earth which now form the 

 great ocean basins date from an early geological epoch, and, consequently, during the 

 period occupied by the dej)osition of the Jurassic, the Cretaceous, and the Tertiary forma- 

 tions at least, the greater part of the surface of the earth has been covered by a sea. 

 As the physical conditions of the world have apparently remained during that time 

 much the same, there seems to be no special reason to doubt that the mean depth of 

 the sea has been throughout about 2500 fathoms, and the temperature of its abyssal 

 region 32° to 40° F., as at the present day. 



VI. The belts of shallower water, whose history during the Jurassic, the Cretaceous, 

 and the Tertiary periods is known to us through local upheavals, slow or rapid, which 

 have brought almost a continuous series of their records into view, show a fauna, 

 difi"erent certainly from the shallow-water fauna of the present day, but comparable 

 with it in- every respect. The records of the abyssal fauna of these past times are 

 naturally more scanty, seeing that they are still mostly beneath the sea; but occasional 

 disturbances have given sufiicient evidence that the abyssal region of the ocean has been 

 inhabited throughout. 



VII. The existing abyssal fauna, including many characteristic animal forms, of 

 which the few mentioned above are perhaps among the most striking, does not appear 

 to bear as a fauna any direct genetic relation to the faunae of shallower water, and seems 

 to be to a great degree independent of the distribution of temperature, due to direct solar 

 radiation or to surface currents. 



VIII. The recent abyssal fauna has a relation to the deep water fauna of the Oolite, 

 the Chalk, and the Tertiary formations, so close that it is difficult to suppose it in the 

 main other than the same fauna which has been subjected to a slow and continuous 

 change under slightly varying circumstances according to some law, of the nature of 

 which .we have not as yet the remotest knowledge. 



It has been discovered within the last few years through a series of investigations, of 



