256 The Naturalist in N icaragua 
area was concerned. - When the land again rose from below 
the sea, the marine fauna was not destroyed—it simply 
retired farther back. 
There is every reason to believe that the production of 
species is a slow process, and if fresh-water areas have not 
continued as a rule through long geological periods, we can 
see how variation has been constantly checked by the destruc- 
tion, first in one part, then in another, of all the fresh-water 
species; and on these places being again occupied by fresh 
water they would be colonised by forms from other parts of 
the world. Thus species of restricted range were always 
exposed to destruction because their habitat was temporary 
and their retreat impossible, and only families of wide dis- 
tribution could be preserved. Hence I believe it is that the 
types of fresh-water productions are few and world-wide, 
whilst the sea has mollusks innumerable, and the land great 
variety and wealth of species. This variety is in the ratio of 
the continuity of their habitats in time and space. 
It follows also, from the same reasoning, that old and 
widespread types are more likely to be preserved in fresh- 
water areas than on land or in the sea, for the destruction of 
wide-ranging species is effected more by the competition of 
improved varieties than by physical causes; so that when 
variation is most checked old forms will longest survive. 
Therefore I think it is that amongst fishes we find some old 
geological types still preserved in a few of the large rivers of 
the world. 
To illustrate more clearly the theory I have advanced, I 
will take a supposititious case. In the southern states of 
America there is reason to suppose that since the glacial 
period there has been a great variation in the species of the 
fresh-water mollusk genus Melania, and in different rivers 
there are distinct groups of species. Now let us suppose 
that the glacial period were to return, and that the icy cover- 
ing, gradually thickening in the north, should push down 
southward as it did once before. The great lakes of North 
America would be again filled with ice, and their inhabitants 
destroyed. As the ice advanced southward, the inhabitants 
of one river-system after another would be annihilated, and 
many groups of Melania entirely destroyed. On the retreat 
of the ice again the rivers and lakes would reappear, but the 
