286 BRANCHIFEROUS MOLLUSCA. 



in its operation, than ardent spirits. Aristotle was aware of 

 this fact, for the Purpurae, he tells us, died in the course of 

 one day in fresh water, although they could live fifteen days 

 under the open atmosphere. But the poison works in gene- 

 ral much more rapidly than is here stated, and we have rarely 

 found a truly marine species resist its influence above a few 

 minutes, some not as many seconds. You will perceive in 

 this fact the ordained means of retaining these animals, in 

 general carnivorous, within their natural parks, if I may so 

 speak, and the surest of all guards against their inroads on 

 the defenceless tribes which inhabit large rivers and lakes. 

 But to occupy the ground that lies between these two tribes 

 the marine and lacustrine we find another race created, 

 whose delight is to dwell in brackish water, 



" Or on the beached margent of the sea," 



and who can bear, with comparative impunity, either the 

 river or the purer sea. Several Littorinaa and Rissose are in 

 this condition ; as well as the common mussel, the cockle, 

 and some littoral Tellinides. The Potamides, a marine 

 genus, is often found at the mouths of rivers ; and Rang has 

 found at the isle of Bourbon, in a fresh water tank, not far 

 removed, however, from the shore, Pintadines and an Aplysia 

 living in society, under stones, with Neritinae and a Melania.* 

 In South America, near Rio de Janeiro, Mr. C. Darwin found 

 a Limneus in great numbers in a lake into which, the inha- 

 bitants assured him, the sea annually, and sometimes oftener 

 entered, and made the water quite salt. M. Gay has stated 

 that he found, in the neighbourhood of Rio, the marine 

 genera Solen and Mytilus, and the fresh water Ampullariae, 

 living together in brackish water. -j- Facts of this kind ought 

 to be kept in remembrance by the geologist, but yet not 

 with the fixedness of gaze that would rather blind than 

 guide him. 



And apropos to this hint: In some strata of compara- 

 tively recent formation, marine and fluviatile shells have 

 been found commingled, and some have made a noise about 

 the discovery as if it were one which led to important deduc- 

 tions ; nor need we dispute its value. Beudant was induced 

 by the discovery to institute some experiments with the view 

 of determining whether or not marine mollusca could not, 

 by adopting proper precautions, be gradually habituated to 

 fresh water and enabled to live in it, and if fluviatile mol- 

 lusca could not, on their side, be accustomed to the sea ; and 

 his experiments incline him to think this might be done, 



* Manual, p. 47. f Darwin's Journal, iii. 24. 



