102 SoLitas—On the Origin of Freshwater Faunas. 
they are remarkably rare. The actively-moving Crustacea sometimes appear to 
push their way up stream, as is shown by the occurrence of two species of freshwater 
Peneus, one inhabiting the Sutlej, at the foot of the Himalayas, the other (P. 
Braziliensis) occurring far up the rivers of North America (Semper, p. 437). Some 
freshwater crabs also appear to be late immigrants into freshwater areas. Of freely- 
moving molluses scarcely any good instances of direct immigration can be adduced. 
Had our streams become populated by direct colonization from the sea, one 
might fairly expect to find more frequent cases of the process still in progress. 
Marine forms becoming estuarine, and estuarine passing into fluviatile forms, 
should be processes of common occurrence ; as a matter of fact, they are so rare 
that one is led to suspect that the Peneus of freshwater streams has been derived 
rather from descending lacustrine species than ascending marine ones. It would 
appear that some very serious obstacle besides those already suggested must exist 
about the debouchment of rivers to hinder the inland progress of marine animals: 
it is possible that in some cases this is of the nature of a thinly-peopled zone or 
desert margin, which none but swiftly-moving animals, such as fish or Crustacea, 
find it possible to traverse. Such a desert might well be produced by the com- 
mingling of the fresh and of the muddy water of a river with the sea. Such rivers 
as I am acquainted with are remarkably bare of vegetation along their seaward 
banks, and the fatal action of salt water on many of the minute organisms they 
bear to sea would probably produce unwholesome conditions extending over a con- 
siderable distance. Again, the percentage of salt may decrease too rapidly from 
sea to river to permit individual animals to cross from one to another in the course 
of a lifetime, though one would not rest much on this, as Semper mentions oysters 
which flourished at the mouth of a river where they were alternately bathed, 
according to the state of the tides, with fresh and salt water; and if oysters are 
capable of withstanding such rapid alternations of medium, no doubt other locomo- 
tive molluscs may be also. Direct immigration appears less probable than the 
second alternative already suggested; and I would rely for an explanation of the 
existence of freshwater animals on those wide changes in the distribution of land 
and sea, which we know to have taken place in the course of geological time. 
The conversion of continental seas swarming with various kinds of life into ter- 
restrial areas, diversified by extensive systems of lakes, has occurred not once or 
twice only in the history of the earth. As lakes began to be produced by a rising 
of the sea-floor, they would probably remain in connexion at first with the open 
sea, and rivers discharging into them would only slightly sweeten their waters ; 
but as elevation slowly continued, connexion with the sea would be severed at one 
point and another, till eventually it would cease altogether, except in so far as 
rivers might flow into the sea from the lake. The waters of a lake so formed 
would continually freshen at a rate depending on the size of the lake and the area 
and rainfall of its catchment basin. 
