326 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 



