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 
of 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. 
