3 2 6 
THE TANGANYIKA PROBLEM. 
plication of new forms in the ocean and the extinction of 
old ones. 
No recognised cause, such as the struggle for existence 
between different types, could be assigned for these three 
synchronous phenomena occurring as they have done. But 
it was shewn that there has gone forward in the sea itself, a 
physical change which there is direct experimental evidence 
to shew would be competent to produce metamorphoses 
of just this kind in a host of diverse animals living in it. 
The gradually increasing salinity of the ocean could act as 
an agent whereby some forms would be definitely killed 
out, some restricted to the fresh-waters of the globe, while 
the changed conditions produced by the increasing salt, 
might act as a stimulus towards variation, thereby pro- 
ducing a host of new forms, such as that witnessed 
among the ammonites at the close of the secondary and 
the beginning of the tertiary rocks. 
Taken in conjunction with other matters, such as those 
referred to by Professor Sollas, respecting the impossibility 
of the free swimming larvae of numbers of marine 
organisms ever getting from the ocean into fresh-water ; 
the hard conditions of fresh-water emphasized by Semper ; 
and the fact that some animals, such as prawns, can, and 
have successfully at all times, colonized the fresh-waters of 
the land from the sea-coast ; the view brought forward was 
found to be capable of giving a tenable and satisfactory 
explanation of the very remarkable and constant peculiarities 
which the fresh-water faunas of the globe present. 
These matters were however discussed, as I have 
already expressly stated, merely in order that some clear 
grasp of the nature and peculiarities of fresh-water faunas, 
and the probable meaning of these peculiarities, might be 
obtained before attempting to attack the special problem 
