Sir Arthur Thomson 263 



the famous voyage of the Challenger (1872-6) for the dis- 

 covery of the new world of the deep sea. 



During this cruise the Challenger, with Darwin aboard, 

 covered nearly seventy thousand miles, and raised 

 treasures of life from the depths of almost every ocean. 

 There are no plants in the great depths, there are no 

 bacteria, but there are sea animals in wonderful pro- 

 fusion and variety, from huge cuttle-fish down to dainty, 

 fragile organisms such as the so-called Venus flower- 

 basket. Living in utter darkness, many of these creatures 

 have developed lights of their own, and glide along lit up 

 like little ships. Of life in the deep sea Sir Arthur says : 



It has been of value to mankind practically in connexion 

 with laying cables ; intellectually, for it has been an exercise 

 ground for the scientific investigator ; emotionally, for there is 

 perhaps no more striking gift to the imagination than the 

 picture which explorers have given of the eerie, cold, dark, 

 calm, silent, plantless, monotonous, but thickly peopled world 

 of the deep sea. 



The flounder, originally a sea fish, is often found some 

 distance up fresh- water rivers. For some reason of its 

 own it is learning to live in fresh water, yet it has to 

 return to salt water to spawn. There are other fish, such 

 as salmon, sea-trout, and shad, which can live either in 

 salt water or fresh, and here we have a clue to the first 

 peopling of the fresh waters from the sea. Either sea fish 

 behaved as the flounder is now behaving, or perhaps an 

 arm of the sea was cut off by a rise of the land and so 

 became an inland lake. This, by the inflow of streams, 

 would become first brackish and at last fresh, but the 

 process would be slow enough to enable its inhabitants to 

 become accustomed to the new conditions. Lake Baikal, 

 in Asia, is an immense distance from the sea, and is now 

 fresh water. Yet seals inhabit it, and seals are marine 

 animals. Here is proof that Baikal was once part of the 



