NATURE AND ORIGIN OF FRESH- WATER ORGANISMS 361 
entirely fresh. Together with typical fresh-water types such as 
tadpoles and gnat larvae, he found quite equally common a species 
of Ml/sis, a Polychaete worm (Nereis or nearly allied form), and a 
small Hydromedusan which he has named Halmomises. A striking 
feature of this case is that these truly marine types appear to flourish 
better in the fresher than in the brackish water, for in the latter 
only occasional specimens were found. 
Additional facts recorded by von Kenne],^ concerning the fauna 
of the river Ortoire in Trinidad, have special significance as indicating 
the manner in which a river may be directly colonised by animal forms 
from the sea. In the wide estuary of this slowly flowing river, the 
tide makes itself felt for miles above the mouth, and, having but a 
very languid current to contend against, is enabled to carry up 
certain marine animals, some of which, being capable of withstanding 
the increased freshness of the water, have settled down permanently 
at considerable distances from the sea. The following forms are 
mentioned as having been found more than eight miles from the river- 
mouth, apparently perfectly adapted to life in fresh water : a species 
of mussel {Mytilus)^ a species of Pholas^ and a Polychaete worm. 
Nor is there wanting certain experimental evidence on this question 
of change of medium. Beudant experimented with a series of 
marine molluscs (he included the Cirripede Balanus), which he 
attempted to gradually accustom to living in fresh water. By a , 
sufliciently slow addition of fresh water, he obtained at last a number 
of different forms living on, apparently uninjured, in water which was 
perfectly fresh, although other species had succumbed in the process. 
In the converse of this experiment, which consisted in accustoming 
fresh- water molluscs to water increasingly salt, very similar results 
were reached. It was thus abundantly proved that a number of 
molluscan species (and Balanus) could live undisturbed in either sea- 
water or fresh. 
But while laying emphasis on the fact that the freshness has not 
prevented representatives of most diverse classes from colonising 
inland waters, we have intentionally disregarded certain cases in 
which this freshness does appear to constitute an impassable barrier. 
We know from experimental evidence, and we infer from cases like 
those in Bengal and Trinidad, that a number of animal types, at all 
events, are extremely sensitive to changes in salinity, and cannot 
survive more than a slight variation in this respect. Why this is so 
in some cases and not in others, we are rather at a loss to explain. 
Whether this character has been acquired by more specialised types, 
and not by lower and more generalised ones, we can only guess ; but 
1 Op. cit, p. 274. 
2 Vide Semper, op. cit., p. 153. 
