10 FRESH-WATER BIOLOGY 



The aquatic life of a permanent fresh-water body is variable 

 within certain limits of time and space. Each season witnesses 

 the coming and going of certain types which are active only in 

 definite periods and by resting spores, gemmules, or eggs bridge 

 over the intervening time. This known seasonal succession is so 

 definite that it gives the life of fresh water a changing character 

 as clear if not as conspicuous to the eye as that on land. One 

 may readily confuse with seasonal succession (i) the numerical 

 variation of a species or group due to favorable or unfavorable 

 conditions, and (2) the migrations which alter vertically or hori- 

 zontally through various water levels the distribution of a given 

 organism. 



One can demonstrate also a stratification of aquatic organisms 

 of at least two types: vertical, as when different species are found 

 to occur within definite limits of depth, and horizontal, as when 

 species are confined to particular regions of streams or lakes. Such 

 relations are discussed fully elsewhere. 



Peculiar types of aquatic environment, such as elevated lakes, 

 saline lakes, and underground waters, have each special types of 

 living organisms. Some of these special environments have been 

 made the objects of extended study which has shown the clear rela- 

 tion of their life to that of other fresh-water bodies of the region 

 while demonstrating at the same time that they present a distinct 

 character of their own (cf. Zschokke, Banta). 



The life of fresh water is probably not original but derived. It 

 came from the sea, by migration through brackish waters or swamps 

 or up into stream systems, by the gradual freshening of marine 

 basins cut off from the sea and converted into fresh-water bodies, 

 by direct transport from one body of water into another through 

 the agency of the wind, on the feet of birds or other wandering 

 animals, and finally by invasion from the land direct. Perhaps 

 the bottom forms came first, as conditions there were first established. 

 Certainly the plankton forms found no opportunity for existence 

 in the violent instability of a young stream. At present the shore 

 forms are the most abundant and the most varied. 



In some deep lakes has been found a peculiar bottom fauna, 

 designated as the fauna relicta, which is composed of types unlike 



