ENVIRONMENTAL FACTORS 



295 



less possibilities of special differentiation under the influence of the 

 environment determine an immense wealth of species of animals, and 

 this is further increased by very decided isolation. Standing waters are 

 often entirely isolated from each other and, with their tributaries, often 

 form separate systems whose contained life is connected only by pas- 

 sive or accidental distribution. Running waters, to be sure, are con- 

 tinuous over greater areas, but the individual river systems are 

 separated from each other by watersheds. And even though they all 



Fig. 78.— Ceratium hirundinella, small and large forms from the same lakes 

 in Switzerland: 1, from the Untersee; 2, from Lake Lucerne; 3, from Lake 

 Maggiore; 4, from Lake Como. After Bachmann. 



empty into the ocean, this forms an insuperable barrier for most forms 

 because of its salt content. 



The wealth of species in the inland waters differs from that of the 

 ocean. In the ocean there are much more numerous structural plans 

 within which the species are invariable over wide ranges. In inland 

 waters the number of types is limited, but within individual species 

 an almost unlimited variation is the rule, at least among the less vagile 

 forms, so that one might almost say, as the number of lakes, so the 

 number of varieties. Since this variability of animals in the inland 

 waters, especially in fresh waters, is so very characteristic, several 

 examples may be added here to those mentioned on p. 83 ff. 



Ceratium hirundinella (Fig. 78), one of the armored flagellates, has 

 three and four horned forms, and the size varies between the extremes 

 of 92/* in Lake Como and 707/x in Lake Schwendi. List 12 found that 

 every pond had its definite local form which is characterized by the 



