THE TANGANYIKA PROBLEM. 23 



water of Lake Shirwa contains, but, following the same line 

 of investigation, I distributed while on Tanganyika five 

 hundred young prawns, in equal quantities of water, among 

 twelve vessels. Leaving two unaltered as a control to the 

 rest, salt was added so that the solution in the remaining 

 ten was increased daily in the proportion given in the table 

 on p. 24 in which the course of the experiments can 

 be seen ; and from which it will be obvious that in all 

 cases the prawns died when the quantity of salt in 

 solution in the vessel reached '8 per cent., no matter 

 what the rate at which the increase had been made ; 

 and some time before this point, although the prawns 

 were plentifully supplied with food, their development 

 was seriously interfered with, the salt producing a 

 progressively stunting action proportionate to its increase. 

 The effect of introducing salt into the environment of fresh- 

 water organisms such as prawns leads thus to exactly the 

 same result as that obtained with extreme slowness by 

 nature in the case of Lake Shirwa. In both, but more 

 especially in the former case, we have direct evidence that 

 fresh-water organisms can stand a certain amount of salinity, 

 but beyond a certain point, which is probably fixed within 

 very narrow limits for any particular type, they are sud- 

 denly killed off. When anywhere near this point has been 

 reached, the salt acts as a poison, and, just as is now known 

 to be the case with the action of a large number of poisons, 

 not much effect is produced, until what may be termed the 

 critical dose has been reached. But beyond this, wide phy- 

 siological disturbances, or death, immediately occur. There 

 is, however, another aspect of change in the environment of 

 organisms like that produced by increased salinity which, in 

 the present connection, it is very important to consider. 

 It has been experimentally shown by a number of investi- 



