Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 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
Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 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- 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 lake35 (Fig. 6). Each of the large lakes of North America has its own forms of one
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