CONDITIONS OF EXISTENCE 



49 



becomes intermittent. This principle is illustrated by an age 

 series of ponds at the south end of Lake Michigan. These are 

 similar in size and age increases in order back from the lake. 



TABLE VIII 



Showing the Number of Entomostraca in Approximately 90 Liters of 



Water (After Shelford) 



Here the number of Entomostraca is greater in the older ponds though some irregularities occur, 

 related to the amount of rainfall. In rainy seasons the increase with age appears almost throughout 

 the series- 



* Intermittent ponds which show irregularities. 



TABLE IX 



Showing Ratio of Number or Quantity of Different Organisms when 



THE Maximum is 100 (After Shelford) 



Rooted vegetation 

 Entomostraca .... 



Midge larvae 



Sphaeridae 



Gilled snails 



Lunged snails 



Amphipoda 



Crayfishes 



Insects 



Fish 



Relative age of ponds 



32 

 80 



20 

 10 



5° 



10 



40 



100 



60 



35 

 80 



5° 

 5° 

 5° 

 90 



S° 

 90 



87 



TOO 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 100 

 87 



The Entomostraca are rated on the basis of actual count of six collections. The other figures are 



estimates. 



In passing from younger to older ponds an increase is noted in 

 the number of animals, excepting fish. These appear to decrease, 

 probably because of the increasing unsuitability of the ponds as fish 



