88 Sottas—On the Origin of Freshwater Faunas. 
very exceptional circumstances could they learn to accustom themselves to a fresh- 
water mode of life; and no doubt we have here one cause, and that a potent one, 
for the exclusion of many marine organisms from our freshwater streams and lakes. 
But it is by no means the only cause, and probably not the chief; for Beudant, 
to whose observations Semper has recently called attention, has already shown 
that several species of marine molluscs can be brought to live in fresh water, if only 
the sea water in which they are at first placed be freshened with sufficient slow- 
ness. Thus in three months he was able to people a freshwater tank with limpets 
and mussels, which then might have been seen living in strange association with 
such true freshwater molluscs as Limnea and Paludina (v. Semper, /. c., p. 153). 
While Beudant was thus successful in overcoming the repugnance of some 
marine forms to a change of medium, he failed with others: of fifteen marine 
species twelve bore the change of medium, and three perished. But it is very 
possible that, under more favourable conditions, these would have yielded to treat- 
_ ment like the rest. An unfailing supply of appropriate food and a secular slow- 
. ness in change of medium would seem to be the two conditions essential to success. 
These given, it is quite possible that all marine animals are susceptible of a fresh- 
water existence. A few considerations will serve to make the probability of this 
apparent. That the sea is the fertile mother of all life was a poetic fancy which 
has now become a fair deduction from admitted facts. Indeed, all naturalists are 
now agreed that all freshwater animals have descended, directly or indirectly, from 
marine ancestors; so that the adaptation in question must have occurred at some 
period in the past history of all freshwater races. But some of these are of such 
a character that it is difficult to admit adaptability for them and to deny it to 
others. If the freshwater Hydra and Cordylophora have survived a change of 
medium, it is difficult to see what reason can be adduced against this being pos- 
sible in the case of other Hydroid polyps. 
Again, so large a proportion of water enters into the composition of all 
‘‘jelly-fish” that they might be regarded as singularly unlikely to survive a 
change of medium; and indeed some experiments of Mr. Romanes prove an ex- 
treme intolerance on the part of marine Medusz in this respect. No one conse- 
quently would have expected to discover Meduse living in fresh water in a state 
of nature, and yet the occurrence of Limnocodium in the tanks of the Botanical 
Society of London, associated with Victoria regia, proves that there is nothing in 
their nature absolutely incompatible with such a mode of existence. The change 
from one medium to another has here taken place, and so profoundly have the 
tissues of Limnocodium been modified in consequence, that it is now more suscep- 
tible to the fatal action of salt water than its marine relations are to fresh. 
G. Romanes remarks, on closing his account of experiments on Limnocodium, 
as follows :—‘‘ If an animal so exceedingly intolerant of fresh water as is a marine 
jelly-fish may yet have all its tissues changed so as to adapt them to thrive in 
