POISONS. 253 
than is fresh water to the unmodified. There can 
be no doubt that the modification was gradual— 
probably brought about by the ancestors of the 
fresh-water Medusa 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 anywhere 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 Royal Society by Mr. A. G. Bourne, 
describing the hydroid stage of the fresh-water Medusa (Proc. 
Roy. Soc., Dec. 11, 1884). He has discovered the hydroids on the 
roots of the Pontederia, which have been growing in the Lily- 
tank for several years, and which are therefore probably the 
source from which the tank became impregnated with the 
Medusee. 
