GENERAL INTRODUCTION TO THE ZOOLOGICAL REPORTS. 



47 



depths beyoud a thousand fathoms were at auy time greatly different from those of the 

 present day. During the Silurian period, as now, and extending continuously from 

 that early time to the present day, a eontinuous ocean with a mean bottom temperature 

 oscillating about the freezing-point, has in all probability covered the greater part of the 

 earth. During all this time an abyssal fauna, of whose existence we have evidence in 



every rescued page of geological history, must have migrated continuously, becoming 

 slowly changed during the lapse of immeasurable time with the slightly altering phases 

 in the distribution of sea and land. It seems only natural that migration through so 

 great a lapse of time, over an area under such uniform conditions, should have become at 

 length universal, and that a singularly uniform fauna should Imve been the result. 



Fig. 24. — CorypJicenoides. A deep-sea fish belonging to tke MacruridiE. 



Although the cold water which occupies the deeper portions of the ocean-basins is 

 comparatively still, it is by no means entirely so, and its movement, although almost 

 secular in its slowness, has a most important influence in securing the permanence of its 

 own temperature, and in maintaining the uniformity of its fauna and determining the 

 direction of the migration of the animals included therein, 



A cold indraught passes from the polar seas northwards and southwards over the bed 

 of the ocean. This we can scarcely doubt, for in all parts of the world where deep tem- 

 perature-soundings have been taken, from the Polar circles to the equator, the temperature 

 sinks with increasing depth ; and it is lower at the bottom than the normal temperature 

 of the crust of the earth, an evidence that a constantly renewed supply of cold water is 



