Introduction 



Almost simultaneously with the appearance of 

 Darwin's great work, there occurred another event 

 which did more perhaps than any other single thing to 

 bring about the recognition of the limnological part of 

 the field of biology as one worthy of a separate recogni- 

 tion and a name. This was the discovery of plancton 

 — ^that free-floating assemblage of organisms in great 

 water masses, that is self-sustaining and self -maintaining 

 and that is independent of the life of the land. Lilje- 

 borg and Sars found it, by drawing fine nets through 

 the waters of the Baltic. They found a whole fauna 

 and flora, mostly microscopic — a well adjusted society 

 of organisms, with its producing class of synthetic 

 plant forms and its consuming class of animals; and 

 among the animals, all the usual social groups, herbi- 

 vores and carnivores, parasites and scavengers. Later, 

 this assemblage of minute free-swimming organisms 

 was named plancton.* After its discovery the seas 

 could no longer be regarded as "barren wastes of 

 waters"; for they had been found teeming with life. 

 This discovery initiated a new line of biological explora- 

 tion, the survey of the life of the seas. It was simple 

 matter to draw a fine silk net through the open water 

 and collect everything contained therein. There are 

 no obstructions or hiding places, as there are every- 

 where on land; and the fine opportunity for quantita- 

 tive as well as qualitative determination of the life of 

 water areas was quickly grasped. The many expedi- 

 tions that have been sent out on the seas and lakes of 

 the world have resulted in our having more accurate 

 and detailed knowledge of the total life of certain of 

 these waters than we have, or are likely to be able soon 

 to acquire, of life on land. 



Prominent among the investigators of fresh water Ufe 

 in America during the nineteenth century were Louis 



*Planhtos = drifting, free floating. 



