60 EVOLUTION 



forms, the pupa which should give rise to 

 the former may be made to give rise to the 

 latter, or the pupa may be affected by cold 

 or by heat so that what emerges resembles 

 not the local form of the species, but a 

 northern or southern variety. Perhaps the 

 most important general result from our 

 present point of view is that "the differences 

 effected by changes in the environment have 

 been shown in some cases to resemble the 

 kind of differences that separate species from 

 each other.' This is suggestive and im- 

 portant, though it does not by any means 

 prove that species have arisen in this way. 



Mr. J. T. Cunningham put very young 

 flounders in an aquarium lighted from be- 

 low, and observed that as they underwent 

 their peculiar metamorphosis the pigment 

 first disappeared as usual from the down- 

 turned side, and then (in 11 cases out of 13) 

 reappeared under the unusual stimulus of 

 light from below. This shows that the normal 

 absence of pigment on the down-turned side 

 of a flat-fish is due to the absence of the light- 

 stimulus in each individual case. 



Some forty years ago Schmankewitsch 

 made a study of a natural experiment that 

 occurred in a salt lagoon which was divided 

 by a dam into an upper and a lower part, the 

 latter the salter of the two. In a spring flood 



