EXTERNAL INFLUENCES AS CAUSES OF VARIATION 283 



The small crustaceans Artemia salina and A. milhausenii 

 have been recognized as distinct, the former living in brackish 

 and the latter in still more concentrated waters, the two differ- 

 ing mainly in the number and length of bristles borne at the 

 extremity of the caudal fins. 



Early in the seventies Schmankewitsch published an account 

 of the mutual conversion of each form into the other, but the 

 facts as given by him have been greatly overstated, as frequently 

 happens in repetition. They are sufficiently significant as first 

 reported, and it seems well to give the original statement as 

 recorded by Bateson, 1 which is as follows : 



The salt lagoon, Kuyalnik, was divided by a dam into an upper and a 

 lower part ; the waters in the latter being saturated with salt, while the 

 waters of the upper part were less salt. By a spring flood in the year 1871 

 the waters of the upper part of the lake swept over the dam and reduced 

 the density of the lower waters to 8 Baume' (= about sp. g. 1.051), and 

 in this water great numbers of A. salina then appeared, presumably having 

 been washed in from the upper part of the lake or from the neighboring 

 salt pools. After this the dam was made good and the waters of the lower 

 lake, by evaporation, became more and more concentrated, being, in the 

 summer of 1872, I4B (about sp. g. 1.103); in l8 73> l8 B (about sp. g. 

 1.135) ; in August, 1874, 23.5B (about sp. g. 1.177), and later in that year 

 the salt began to crystallize out. In 1871 the Artemia [as first carried 

 over] had caudal fins of good size, bearing eight to twelve, rarely fifteen 

 bristles, but with the progressive concentration of the water the genera- 

 tions of Artemia progressively degenerated, until at the end of the summer 

 of 1874 a large part of them had no caudal fins, thus presenting the 

 character of A. milhausenii. FISCHER AND MILNE-DWARDS. 



Bateson adds : 



A similar series was produced experimentally by gradual concentration 

 of water, leading to the extreme form resembling A. milhausenii. It was 

 found also that if the animals without caudal fins were kept in water which 

 was gradually diluted, after some weeks a pair of conical prominences, 

 each bearing a single bristle, appeared at the end of the abdomen. 



The experimenter also relates that by breeding salina in still 

 more diluted water he attained a form resembling Schaffer's 

 genus Branchipus. But the principal difference between the 

 genera is that in Artemia the last segment is about twice as 



1 Bateson, Materials, etc., p. 96. 



