20 Fresh Water Oligochceta. [zoe 



66. Glyceria fluitans R. Br. Denver. 



67. Glyceria pallida Trin. Denver. 



68. Glyceria grandis Watson. Denver. 



69. Agropyrum glaucum R. & S. var. occidentale 

 V. & S. The Common Blue-Stem. Denver. 



70. Agropyrum tenerum Vasey. Denver. 



71. Elymiis Virginiais I,, var. sub?)iuticus Hook. Denver. 



RESTRICTED DISTRIBUTION OF FRESH WATER 



OLIGOCH^TA. 



by gustav eisen. 



The geographical distribution of fresh water oligochaeta, as 

 compared to fresh water algae, is most interesting and unexpected. 

 It is well known that a majority of species of fresh water 

 algae are cosmopolitan, and even locally widely distributed, 

 being rarely confined to special localities, such as a single pond, 

 spring, or lake. An alga which is found in one spring or creek 

 is almost certain to be found in some other spring or creek in the 

 vicinity. Many species have a world-wide distribution, while 

 others of rare occurrence have been found in distant localities. 

 With the fresh water oligochaeta this manner of occurrence is 

 exactly opposite. Few species are found in countries far apart. 

 Not one species is found distributed all over the world, while by 

 far the greatest number of species are endemic in certain districts, 

 or even confined to certain ponds, lakes, rivers, creeks, or springs 

 outside of which they do not appear to thrive. With the genera 

 this, of course, does not hold good. True, I,umbrici are found the 

 world over, but it is more than probable that whenever the same 

 species is found in very distant countries, it has been artificially 

 introduced there with economic or garden plants brought along 

 by nursery men or horticulturists. The distribution of fresh 

 water oligochaeta is as yet only imperfectly known, and it is 

 too early to compile their geographical distribution, but enough 

 is known to warrant us to believe that there are some powerful 

 influences in nature which operate on and curtail their 



