THE TANGANYIKA PROBLEM. 29 



I. That there is progressing this change with respect to 

 salinity. 



II. That it can be shown experimentally that such a 

 change after it reaches a certain degree is both prejudicial 

 to some organisms and causes wild variation in others. 



III. That these tendencies are competent to bring about 

 the separation of marine from fresh-water faunas, the latter 

 arising all over the world from the general marine fauna at 

 about the same time. 



IV. That the facts of the morphology and distribution of 

 the existing fresh-water faunas independently point to some 

 such cause as having operated towards their production in 

 the past. 



Accepting these various indications respecting the manner 

 in which fresh-water faunas have become differentiated, it 

 would appear that the present-day fresh-water faunas are 

 to be regarded as chiefly composed of the remains of a 

 once widely distributed and ancient sea-fauna, the ancestors 

 ol the surviving components of which were forced out 

 of the ocean into the fresh- waters of the globe owing to a 

 change in the character of the sea itself. This change ap- 

 pears to have become sufficiently strongly marked to have 

 produced an appreciable differentiation at a period roughly 

 corresponding to the commencement of the formation of 

 the secondary rocks. In this manner it would appear that 

 assemblages of similar organisms have of necessity taken 

 to fresh-water all over the world about the same time, just 

 as one could imagine the modern universally distributed 

 organisms of the marine littoral, such as the Trochoid 

 molluscs and the shore crabs, being simultaneously every- 

 where driven into the fresh-waters of the land if the sea 

 became too hot to hold them. 



