62 EVOLUTION 



disappeared as usual from the down-turned 

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

 appeared 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 hi 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 

 in 1871 the waters of the upper part swept 

 over the dam and reduced the salinity in 

 the lower part. Thereafter great numbers of 

 a tiny brine-shrimp, Artemia salina, were 

 observed in the lower part, having been 

 presumably washed in. After a tune the dam 

 was repaired, the water gradually regained 

 its great salinity, and the brhie-shrimps in 

 the course of their rapid generations lost the 

 well-developed caudal fins characteristic of 

 Artemia salina and became like another 

 form without caudal fins, Artemia mil- 

 hausenii. Passing from observation to experi- 

 ment, Schmankewitsch found that gradual 

 concentration of the water led to the replace- 

 ment of typical forms of Artemia salina by 

 forms like Artemia milhausenii, and he also 



