Marsh — The Plankton of Fresh Water Lahes. 169 



in Europe. In the case of one species it is not only found in 

 Europe, but in As 'a and in xVfrica, and literally does not vary 

 a hair in these widely separated localities. On the other hand, 

 the genus Diaptomus is very variable. We have not a single 

 species which is common to the European lakes. Not only that, 

 but there are many localized species in the United States. One 

 species occurs, so far as known, only in a few lakes in the north- 

 ern part of the lower peninsula of Michigan. A second is 

 widely distributed in all the smaller lakes across the continent 

 in the northern states. Another goes from the center of Wis- 

 consin north into the Arctic regions. In the Rocky Mountain 

 regions are several peculiar species. Through the southern 

 states two species are found which never come north. Mexico 

 has at least one peculiar species. Of the other organisms, both 

 animal and plant, most are world-wide in their distribution. 

 Erom this fact of the general uniformity of fauna and flora over 

 wide regions, it is clear that the study of a lake which simply 

 produces a farmal and floral list is of very little value. There 

 was a time when such lists were important, before this uniform- 

 ity of distribution was determined, but that time is long since 

 past, and those European authors w-ho continue to fill the pro- 

 ceedings of learned societies with lists resulting from desultory 

 explorations of one or more lakes are almost wasting printers' 

 ink. 



In the littoral region we find usually an abundantflora. Those 

 plants which need an anchorage find it in the mud of the bottom, 

 and the unattached plants are protected by those that are sta- 

 tionary. Protected by these plants and living upon them is an 

 abundant fauna in which Crustacea are the most prominent, 

 although we find great numbers of rhizopods, infusoria, sponges, 

 hydrozoa, worms, true insects and mites, mollusca and bryozoa. 

 This abundance of the lower animals forms a rich supply of food 

 upon which the higher animals can live. It is in this littoral 

 region that the fish get the most of their food, and every fisher- 

 man knows that marshy borders are necessary to maintain the 

 supply of fish. The animals of the littoral plankton are opaque, 

 and generally are so colored that w^hen they are at rest they are 

 inconspicuous. Those that swim about and then drop to the hot- 



