ON MOLLUSCA. 231 
instance eels, and that almost suddenly. Why should not the 
mollusca do the same? However, there is no very positive 
fact to prove this possibility, at least for the same species ; for 
it is not so with the genera. We know, in fact, that some 
species of the same genus can live in the fresh-water, and 
others in the salt-water. We are acquainted, for instance, 
with one species of true mussel in the Danube, and many 
cerites, which are equally found in fresh-water. But if the 
species of mollusca cannot suddenly pass from the salt-water 
into the fresh, may they not do so gradually? Do we not, in 
fact, see in certain ponds, which communicate but rarely with 
the sea, and the saltness of which is diminished by little and 
little by rain-water, mollusca living, which are truly marine, 
and appearing to exercise all their functions there in complete 
perfection? ‘The fact is certain, and has furthermore been 
experimentally proved by M. Beudant. But it is not equally 
certain that animals habitually accustomed to live in the salt- 
water, and which are thus forced by natural or artificial cir- 
cumstances to live in that which is almost fresh, or altogether 
so, can reproduce there. But though not placed out of doubt, 
yet the fact which has been observed of marine and fresh- 
water mollusca living together, in the slightly salt-waters of 
the Gulf of Livonia, is in favour of this opinion, and still 
more that which Nilson relates in his history of the mol- 
lusca of Sweden, namely, that on the coasts of Norway, in 
places where there is no mouth of a river, he has found 
uniones, anodontes, and cyclades, living mixed up with mol- 
lusca of the genera venus, cardium, and cytherea. 
Adanson assures us positively, that during one half of the 
year the Niger has only fresh water, and that nevertheless 
several genera of mollusca are found there, which, during the 
other six months, live in the salt-waters. 
The aquatic mollusca, whether marine or fresh-water, also 
do not live absolutely in the same circumstances: the latter, 
