POISONS. 253 



than is fresh water to the nnmoclified. There can 

 be no doubt that the modification was p-radual — 

 probably brought about by the ancestors of the 

 fresh-water INIedusa penetrating higher and higher 

 through the brackish waters of estuaries into the 

 fresh water of rivers — and it would, I think, be 

 hard to point to a more remarkable case of pro- 

 found physiological modification in adaptation to 

 changed conditions of life. If an animal so exceed- 

 ingly 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 fresh water, and even die 

 after an exposure of one minute to their ancestral 

 element, assuredly we can see no reason why any 

 animal in earth or sea or anywdiere else may not in 

 time become fitted to change its element." * 



* While these sheets are passing through the press, a paper 

 has been read before the Rojal Society by Mr. A. G. Bourne, 

 describing the hydroid stage of the fresh-Avater Medusa {Proc. 

 Roy. Soc, Dec. 11, 1884). lie has discovered the hydroids on the 

 roots of the Pontedcria, which have been growing in the Lily, 

 lack for several years, and which are therefore probably the 

 source from wliich the tank became impregnated with the 

 Medu8a3. 



