PHYSrOGENESIS. 229 



via! Life, to which I refer my readers for a fuller ex- 

 position than can be given here. 



a. Relation of the Size of Siiclls of MoUusca to the En- 



vironment. 



It has been observed that both in natural condi 

 tions and in confinement; shells of fresh-water Mol- 

 lusca grow to a larger size in larger bodies of water, 

 and become reduced in size as the bulk of water in 

 which they live is reduced. Varigny has shown that 

 the reduced size follows a reduction of the air-surface 

 of the water rather than a reduction of the actual bulk, 

 though the two conditions may often coincide. He 

 also shows that, other things being equal, the size of 

 individuals is inversely as their numbers in a given 

 enclosure. 



b. The Conversion of Arteniia Into Bratichinecta.^ 



In 1 87 1 the spring flood broke down the barriers 

 separating the two different lakes of the salt works 

 near Odessa, diluting the water in the lower portion 

 to 8° Beaum^, and also introducing into it a large 

 number of the brine shrimp, Artemia salina. After the 

 restoration of the embankment, the water rapidly in- 

 creased in density, until in September, 1874, it reached 

 25° of Beaum^'s scale, and began to deposit salt. With 

 this increase in density, a gradual change was noticed 

 in the characters of the Artemiae, until late in the 

 summer of 1874 forms were produced which had all 

 the characters of a supposed distinct species, A. muel- 

 hausenii. The reverse experiment was then tried. A 

 small quantity of the water was then gradually diluted, 



lAbstracted from an account by J. S. Kingsley, Statidai-d Natural His- 

 tory, Vol. II. 



