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) 



Relative age of ponds 



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 



