THE TANGANYIKA PROBLEM. 
2 3 
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, sait 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- 
