THE TANGANYIKA PROBLEM. 
27 
salt may crystallise out on the reeds and beaches in glitter- 
ing white precipitates, as I have seen it repeatedly round 
Shirwa and the other salt-pans of the African interior. A 
lake having no outflow bears the same relation to its 
drainage area that the ocean does to the whole land sur- 
face of the earth, the volume of water in the sea remains 
constant, but vast quantities of this water are for ever 
evaporating into the atmosphere, and are then flung broad- 
cast over the surface of the land, from whence they 
eventually drain back in the form of rivers, but each in 
possession of a certain quantity of salt which they add to 
that already in solution in the sea. The sea is nowhere 
nearly saturated with salt, and the above reasoning would 
appear to lead to the conclusion that if we had a sample 
of the ocean which was sufficiently old we should find that 
it contained less salt than does the present-day sea-water ; 
whether, however, we can agree with Hunt that the water 
sometimes enclosed in ancient rocks may be looked upon 
as fossil sea-water, and consequently that the ancient 
oceans were richer in magnesium and potassium than the 
present-day sea, is a matter into which we need not at 
present go ; all that it is important to realise being, that 
it seems to be demonstrable from different sources that 
the sea has changed and is changing in composition with 
a gradual increase of common salt in solution, and that the 
indication of a physical change in the sea which we obtained 
from the purely zoological considerations, discussed in the 
preceding pages, seems to be in unison with various other 
lines of evidence. 
There thus appears to be reason to suppose that there has 
been a slow increase of common salt in solution in the sea, 
and if this is so, it would also appear that the general 
course of marine evolution may have been profoundly in- 
