49-4 DR MURRAY ON THE DEEP AND SHALLOW-WATER MARINE FAUNA 



It may therefore be assumed that the identical species now found living towards 

 both poles, or their immediate ancestors, had a world-wide distribution, which involves 

 a nearly uniform temperature throughout the whole body of ocean waters. From what 

 has been stated with reference to coral reefs, and from what we know of the distribution 

 of plants in the coal period, this appears to have been the actual state of matters during 

 the earlier stages of the earth's history ; down to the middle of Mesozoic times the ocean 

 had, probably, an approximately uniform temperature of about 70° F. from pole to pole, 

 being probably not much warmer at the equator than elsewhere. The evidence afforded 

 by the distribution of fossils in the geological strata, proceeding backwards in time from 

 the most recent to those of Palaeozoic age, indicates that the tropical zone of temperature 

 slowly widens towards the north and south till in the earlier ages it eventually embraced 

 the whole world. 



From the general character of the deep-sea fauna, as well as from its distribution over 

 the floor of the ocean at the present day, it cannot be said that there is any evidence in 

 support of the view that from the Silurian period to the present day there had been, as 

 now, a continuous deep ocean with a bottom temperature oscillating about the freezing 

 point, and that there had always been an abyssal fauna. 1 It is more probable that in 

 early times the ocean had a nearly uniform temperature throughout its whole mass, and 

 that, like the Black Sea at the present day, it was uninhabited much below the mud-line, 

 except perhaps by some species of Bacteria. 



We may suppose that if cooling set in at the poles towards the middle or close of 

 Mesozoic times colder water then descended to the greater depths of the ocean, carrying 

 with it a larger supply of oxygen, so that it then became possible for animals to live 

 in the greater depths, and migrations slowly took place into the deep sea. A cooling 

 at the poles such as here indicated would bring about the destruction of many shallow- 

 water organisms, especially of those provided with pelagic larvse and of those which 

 secreted large quantities of carbonate of lime for their shells and skeletons. Owing to the 

 weeding out of these groups of animals a fauna less rich in genera and species would be 

 left behind ; the survivors would be chiefly those with a direct development inhabiting 

 the deeper mud-line. In this way we may account for the relatively few species in the 

 shallow or shore waters of the polar regions when compared with the number present in 

 depths less than 20 fathoms within the tropics, and for the absence of pelagic larvse of 

 Benthos animals in the cold waters towards the poles. The large number of individuals 

 belonging to many of the polar species compared with what obtains within the tropics, 

 as well as the identity or great resemblance of the species in the two polar areas, may be 

 explained by similar considerations, for in water of a low temperature the metabolism in 

 cold-blooded animals would be much less rapid than in water of a high temperature, and 

 all those changes which result in the evolution of new species would proceed at a much 

 slower rate at the poles than in the tropical belt. 



As the process of cooling proceeded and the Antarctic continent became covered 



1 Wyvillk Thomson, Zool. Chall. Exp., vol. i., Introcl. p. 47. 



