188 Geographical Distribution of the Common Oyster. 
not to interfere with one another. On account of the water 
being less salt (2°9-3°1) the shells are thinner than on the 
natural banks, in accordance with what has been observed 
also in the case of other testaceous mollusks. Through these 
changes the oyster is improved as an article of food and 
commerce ; but the improvement is not attained without ano- 
ther effect, which accompanies artificial fattening of animals 
as commonly as the abnormal increase of the liver does, 
and which is of the greatest importance for the whole 
question of oyster-culture. All the physiological energy 
of the animal being concentrated on the development of the 
individual, another side of its life, its capability for continuing 
the species, is impaired. Several authors have noticed the 
small quantity and limited vital power of the spat produced by 
oysters in artificial parks ; but the fact has not as yet received 
the attention it deserves; it has not been observed that it is a 
regular effect of less salt water and that consequently parks 
for fattening oysters cannot be self-supperting in the long run. 
A diminution of the saltness of the sea amounting to 0°5, 
in connexion with certain other physical circumstances, parti- 
cularly an admixture of fresh water, is consequently enough 
to exercise a notable influence on the development of the 
oyster generally, and especially on its power of propagation. 
This check will increase in effectiveness with the admixture 
of fresh water; and there is a point where the individual oyster 
is still perfectly able to live and thrive, but unable to propa- 
gate the species. Experience shows that oysters are able to 
live long in water of much less saltness, and even attain a size 
and delicacy which could not be reached if any part of its 
vital power were to be spent on propagation of the species. 
The minimum of saltness compatible with the existence of 
oysters has not yet been determined ; and the circumstance that 
the animal is very susceptible to cold, if the saltness of the 
water decreases, renders experiment on this point very difficult. 
Von Baer puts this minimum at 1:7; but propagation is then 
out of the question. In several instances different banks in 
the same locality have been found to exhibit great differences 
with regard to fertility; and it has been suggested that the 
reason might be that the products of the generative organs do 
not come to maturity in allindividuals at the same'time. But 
it is probable that their more or less favourable situation in 
regard to the access of salt water is of not less importance in 
the matter—particularly as the degree of saltness of the water 
would probably show its effects not only in increasing or dimi- 
nishing the general fertility, but also in accelerating or delay- 
ing the maturity of the secretions in question. 
