THE TANGANYIKA PROBLEM. 
I 5 
life — (i) a universal series of types, and (2) another com- 
posed of organisms which only occasionally appear in fresh- 
water, and are related to a particular district or to a lake. If 
now we examine the character of the animals that form the 
types of the first series, we are confronted with this very 
curious and remarkable feature characteristic of them all. 
They have no counterparts or close allies in the ocean at the 
present day; and on anatomical examination they are found to 
stand in the relationship of ancestors to numerous well-known 
marine forms. If, on the other hand, we consider the forms 
which are peculiar to the two lakes in question, and more 
especially if we consider the forms similarly peculiar to a 
number of other lakes and distinct fresh-water areas, we 
find that these forms have almost invariably, like the 
prawns in the Lago di Garda, in Italy, close allies in the 
existing marine fauna, and generally exhibit closest affinities 
with some of the organisms which inhabit the sea nearest to 
the particular fresh-water which may be studied. We find, 
in fact, usually in the fresh-waters of the globe a fauna 
which is composed of an ancient and almost universal stock to 
which it appears that there may be added in any particular 
fresh- water a more or less large and variable stock of animals 
which have migrated into this area at one time or other from 
the nearest seas. The origin and nature of this latter series 
is obvious and plain, but the origin and relationships of the 
first — i.e., the ancient and universal series — is not so clear. It 
will, in the first place, therefore, be well to distinguish this 
universal series by a special name, and to speak ot it as the 
primary fresh-water fauna , and, dealing with the second 
series in the same way, to call it the secondary fresh-water 
series. That the types representing the groups consti- 
tuting the primary fresh- water fauna of the world should 
generally exhibit ancestral characters in their organisation is 
