194 THE RIDDLE OF THE BURBOT 



along our south coast into the Atlantic, receiving in its 

 course the waters of the south-flowing rivers. 



With the return of temperate conditions the Rhine 

 was free to resume its northerly course, and to receive 

 once more the waters of the Trent and other easterly 

 flowing English rivers. But possibly not the Thames. 

 That river may have continued to flow through the 

 new channel cut by the torrential outflow from the Rhine 

 dam, in which case, being severed from the continental 

 river system, it could receive from the Rhine no form 

 of aqueous life that could not pass through salt water. 

 It would be restocked with those species of fresh-water 

 fish that had survived the glacial period in the open 

 waters of the relatively warm region which the land ice 

 did not invade. Such rivers, the Medway, the Aran, 

 the Itchen, the Test, the Avon, etc., having never been 

 connected with the Rhine, contained no burbot, which 

 may be the reason for the absence of that fish from the 

 Thames. 



The other easterly flowing rivers, the Yorkshire and 

 the Norfolk Ouse, the Nen and the whole Trent system, 

 when released from the land ice, would cut their way 

 through the northern plain and rejoin the Rhine, 

 whence they would be colonised by the aqueous fauna 

 of that river, including the burbot. 



Geologists have learnt to be very cautious in forming 

 conclusions founded upon the distribution of living 

 fauna, which, of course, affords much less trustworthy 

 evidence than that supplied by fossil remains. Birds 

 and insects are so mobile, mammals so commonly 

 subject to transport and acclimatisation by man, that 



