president's address. 
469 
of resistance of aquatic animals to altered salinity, and the older 
work has been so well summarised in Semper's "Animal Life” 
that it is not necessary to refer to it here. The nature of the 
toxic effect of abnormal concentration or dilution of the external 
medium is, however, still a matter for speculation. That it is 
not merely due to the abstraction or absorption of water is clear 
from the experiments of Loeb and of Ostwald, and it is sug- 
gested by Loeb 1 that the death of marine Amphipods (Allor- 
chestes) when transferred to fresh water is due to the extraction 
of salts which are necessary to their existence. 
Ostwald J has experimented on the toxic effect of sea water 
on fresh water Gammarus, and found that the various salts differ 
in toxicity and either reinforce or counteract each other in effect. 
He found that the salts in the water might be increased or 
diminished very greatly up to a certain point without serious 
injury, but that, once this critical point is reached, death ensues 
very rapidly, possibly owing to the coagulation of proteids. 
We require more knowledge of the influence of the different 
salts in fresh and salt water, since we know that certain of them 
have deep physiological effects. Magnesium salts, for example, 
seem to act generally as depressors of activity, and also to 
neutralise to some extent the toxicity of other salts. They are 
necessary for the growth of young Artemia (Breckner, 1 ) but 
seem to inhibit the growth of the fresh water Daphnia according 
to my own observations. Potassium salts are exceedingly 
poisonous to Daphnia, and also to Artemia, though their effect 
in the latter case is somewhat neutralised by the addition of 
Magnesium salts . 1 On the other hand, even distilled water may 
be toxic for fresh water Gammarus, the toxicity being removed 
by the addition of a small amount of Sodium Chloride . 4 
It is necessary to know the effect of salts, not merely on the 
life of the individual, but chiefly on its growth and reproductive 
power, for the acclimatisation of the individual is useless unless 
it is able to propagate. So far as concerns the survival of the 
1. Pfliiger’s Arehiv, 97. 1903, p. 394. 
2. Ostwald. Univ. of Calif. Pub. Physiol. IX., No. 18, 1905. 
3. Breckner. Sclirift. Nat, Ver. Schl. Holst., 14, 1909. 
4. Bullot. Univ. Calif. Publ. Physiol., 1, 1904. 
