474 
PRESIDENT’S address. 
a sessile species in normal circumstances to extend its range 
further up the river, though it might assist in populating the 
lower reaches. At times of abnormal tides, such as frequently 
occur in our own rivers, the floating fauna would be driven far 
beyond the normal tidal range and might so make good their 
ground. That is no doubt what has happened with regard to 
Neomysis, Balanus, and other species in the Bure. At times 
of abnormally high tides a typical marine plankton may be 
found in the Bure, fifteen miles and more from its mouth, so 
that there would be no difficulty in the larvae of marine animals 
establishing themselves in the river within this limit, so far at 
least as concerns the currents. 
4. Turbidity. 
One further factor which may enter into the problem of immi- 
gration into fresh water is the great turbidity of rivers at their 
mouths. Examination of the water of the lower reaches of the 
Bure, for example, shows that it contains, as a rule, compara- 
tively little animal life, and that the floating Algae and Diatoms, 
so abundant both in fresh water and in the sea, are almost 
entirely absent. On the other hand, the water is heavily 
charged with vegetable debris and with fine mud. Whether 
the latter, as such, has any considerable injurious influence on 
the majority of estuarine animals I am unable to say, but it 
seems probable that it may largely impede the breathing of gill- 
bearing animals. The experiments of Weldon on Crabs from 
the Cattewater at Plymouth 1 seem to prove that the clay 
carried by the river is actually exercising a selective influence, 
so that there is a tendency for these crabs to evolve a more 
efficient straining apparatus to protect their gills. Many 
aquatic invertebrates feed by indiscriminate sifting of material 
suspended in water, and it is clear that when the water is 
charged with an excess of indigestible matter the conditions 
must be most unfavourable for such forms. Turbidity of the 
water must add to the difficulties to be surmounted by immi- 
grants, but is not in itself an obstacle of serious importance. 
1. Rep. Brit. Ass., 1898, p. 867. 
