ON THE NATURE AND ORIGIN OF 
FRESH-WATER ORGANISMS 
By WILLIAM A. CUNNINGTON, M.A., Ph.D. 
It is a well-known fact that the forms of life found in fresh water are 
usually very different from those found in the sea. Everyone is aware 
that carp live in fresh water, and that bladder-wrack is to be found 
on our coasts, and not in rivers and ponds. But the matter goes 
further than this, and a more detailed study shows that there is quite 
a large number of forms perfectly characteristic of fresh water and 
not occurring in the sea, while another series is equally distinctive of 
the ocean and unknown in the fresh waters of the globe. It will 
perhaps be well to consider first the animals (fauna), and afterwards 
the plants (flora), of these two great divisions of aquatic life. 
When we study the general characters of a normal fresh-water 
fauna, and contrast them with those of a marine fauna, we soon 
become aware of an overwhelming preponderance of species in the 
ocean. ^ That is not to say that the waters of rivers and lakes are 
not well stocked, but that they are stocked by a far smaller number 
of different forms, which is not surprising when we consider the 
insignificant total extent of fresh water, compared with the vast size 
of the ocean. 
Amongst the Vertebrata proper, it is principally the fishes which 
have retained an aquatic mode of life. Excluding the wading and 
diving birds, we have a few truly aquatic mammals and reptiles, 
which have undoubtedly acquired this habit of living secondarily. 
The Amphibia, mainly aquatic in early life, are mostly terrestrial in 
their adult state, but, as far as they inhabit water at all, live in 
fresh water and never in salt. Coming to the fishes, then, we find 
that the great majoritv of the known species are inhabitants of the 
sea, the extensive group of the Elasmobranchs being almost entirely 
1 Cf. Quinton, "L'eau de Mer Milieu organiqiie," Paris, 1904. The author 
contrasts (p. 55 et seq.) the orders, etc., represented in the sea, with those known 
to occur in fresh water. 
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