439 



I'autre." It is evident tliat tliis can occur only when siniilnr conditions (e. 

 g., pelagic) are present in tlie different lal<;es. In lakes this similarity may 

 exist in certain parts during the major part of their existence. In iionds 

 the period for this reciprocal reaction is very limited. 



Although a single pond contains relatively few species, all the pouds 

 in an area of several square miles show a much greater variety. 



Many ponds have been examined but detailed dat;i concerning them 

 have not yet been collected. However, the following note will illustrate 

 what is meant. On Jan. 11, 1910, pond No. 1 P contained Cyclops hiciispi- 

 (Jatiis, Chydorus sphaericus, Cypridopsis vidua and alona. Pond No. 2 T 

 contained Cyclops scrrukttus, C. IcncJcarti ; and an unidentified Daphnid. 

 Pond No. G P contained a few Cyclops semilatns and an enormous number 

 of Bosmina coruuta. Other groups of organisms show an equal variety. 



RELATION TO CAVE PIjVNKTON. 



This variety in the fauna of different ponds has an important bearing 

 upon tlie relation of pond plankton to that of caves. That the plankton 

 of the cave streams of this region is derived from certain of these ponds 

 is well established. Only a small niunber of the organisms in any pond 

 are able to withstand the inimical cave conditions. I have never found all 

 the species reported from the Shawnee Cave stream in any one pond. 



These facts indicate that the cave plankton is a composite of such or- 

 ganisms of the contributing pouds as are able to withstand cave condi- 

 tions. It is probable that the greater the number of contributing ponds. 

 the richer will be the fauna at the outlet of the cave stream. 



The relation of these solution ponds to a cave stream is quite com- 

 parable to the relation of backwater lakes, bayous, oxbow cutoffs, etc., to 

 the river in whose valley they lie. Kofoid ('03, p. 546) states concerning 

 the Illinois River, "The plankton indigenous to the channel itself is of 

 small volume as compared with, that contributed from the backwaters." 

 There is, however, this difference. In the cave the processes of groA^-th 

 and reproduction are very much inhibited, while in the river they continue 

 or may even be increased. Kofoid (Ic) has shown that during periods of 

 low water, the river may contain more plankton than the contributing 

 waters. This condition never exists in cave streams and obviously never 

 can exist. 



