FIRST OBSTACLE 171 



will begin with the first, and seek to discover the cause 

 by which marine animals are excluded from the fresh- 

 water world. 



No doubt the very first explanation which will suggest 

 itself to every one will rest on the difference in character 

 of fresh and sea water. The quantity of dissolved 

 material in sea water is certainly considerable, it consists 

 chiefly of sodium chloride or common salt : river water, on 

 the contrary, is almost pure : the precise difference will 

 be seen by inspection of the following table : 



Analysis Analysis of 



of Sea Water, River Water, 



North Atlantic. Thames at Thames 

 Ditton. 



Calcium carbonate 0'004 ... 0-0168 



Magnesium carbonate ... 0*0018 



Calcium sulphate 0140 ... 0-0044 



Magnesium sulphate ... ... ... 0*221 



Potassium sulphate ... 0-0002 



Sodium chloride 2'773 ... 0-0016 



Potassium chloride 0'069 ... 0-0009 



Magnesium chloride ... ... ... 0'344 



Silica 0-0015 ... 0'0009 



Organic matter, &c ... 0"0035 



Total 3-552% 0-0304 % 



Thus sea water contains more than one hundred times 

 as much dissolved material as river water, and as a direct 

 consequence of this marine animals in general quickly 

 succumb when transferred to fresh water, while fresh- 

 water animals when placed in sea water even more 

 rapidly perish. The rule, however, is not without fairly 

 numerous exceptions ; the salmon and its tribe, which are 

 able to pass direct from the sea to rivers, and from rivers 

 to the sea, W 7 ill at once suggest themselves. Indeed, the 

 susceptibility of different kinds of animals to the change 

 of medium would appear to vary within wide limits. In 



