GEOGRAPHIC ISOLATION 



83 



The theoretic conditions for the transformation of species in fresh 

 water are accordingly extremely favorable. The variability of the 

 wide-ranging inhabitants of fresh water is extreme, but is modified by 

 the ability to transgress barriers either by active or passive trans- 

 portation. 



Examples of the multiplicity of variations in fresh-water animals 

 are striking. Thus the genus of mussels, Anodonta, represented by 2 

 species in middle Europe, according to Clessin, was divided into 26 

 species by Kuster, while French students have recently recognized 

 more than 200 for the same area. The forms differ in outline, size, 



Fig. 6. — Summer forms of Bosmina coregoni from various Baltic lakes: 1, 

 Lake Paarstein; 2, Lake Rzumo; 3, Lake Wolzig; 4, Lake Luggewiese; 5, Lake 

 Steinkrug; 6, Lake Dlusitsch; 7, Lake Wolzig. After Riihe. 



thickness of shell (Fig. 5), and in the coloration of their outer and 

 inner surfaces, but they are completely united by intermediate speci- 

 mens. 34 The genus Pwidium, the appropriately named Dreissena poly- 

 morpha, and the snails Limnaea and Planorbis exhibit a similar varia- 

 tion. The lakes of the glaciated area in Europe are inhabited by 

 fishes of the salmonid genus Coregonus, whose variability is such that 

 each well-isolated lake is inhabited by well-defined subspecies of one 

 or more forms, and the degree of divergence is roughly proportional 

 to the distance between the lakes. The species of cladocerans, espe- 

 cially of the genus Bosmina, vary in form from lake to lake 35 (Fig. 6). 

 Each of the large lakes of North America has its own forms of one 



