438 



snrrouncliug trees cover the bottom so that a very clistiiiot but iiiiiforui 

 condition is developed. Besides tlieir limited area, a second cause of this 

 uniformity is their sudden formation. During the summer of '08, four 

 miles east of Mitchell, Indiana, an open sinkhole containing several acres 

 became sealed. With the first rain a pond ^^■ith an area of about one acre 

 was formed. This is the typical phenomenon in the formation of soiution 

 ponds. It may take a very long time for the solution cone or sinkhole to 

 form, but when the opening at its base becomes sealed, the pond reaches 

 its maximum depth quite suddenly. The result is that an a(iuatic habitat is 

 formed with no aquatic fauna. The first forms introduced into a pond at 

 this stage liave no competition and soon take possession of the entire area, 

 thus making it more difficult for a related form to establish itself. An- 

 other difference between these ponds and lakes (which seems to be due to 

 tlieir fundamental difference in size) is the paucity of species in the former 

 n'lien compared with the latter. It seems probable that, other things being 

 equal, the larger the lake, the greater the variety in the fauna. Forel ('04) 

 reports 22 species and 10 varieties of cladocera, 9 ostraccda and 12 gastro- 

 poda from Lake Geneva, while from the smaller Pltiner See, Zacharias 

 ('93) reports 20 species and varieties of cladocera, one ostracoda and 10 gas- 

 tropoda. Burchardt ('00) observes that the plankton of the Alpenacher 

 See contains fewer species than the Yierwaldstiidtersee. Dr. W. Halbfass 

 collected from a numl)er of lakes in northern Germany and sent the ma- 

 terial to Zacharias for examination. The lists show only one cladoceran 

 from Dolgensee bei Neustettin, a very small body r>f water, while Wilmsee, 

 a much larger lakelet, contained six. The fauna! list for any lake that has 

 been explored with care, is much greater than that of this pond. 



This is due to the uniformity of conditions that prevails over the en- 

 tire area of a given pond at a i^articular period, and to the fact that the 

 pond after its formation, changes very rapidly, thus making it suited for 

 a particular si^ecies for a relatively short period. If the species is not 

 Introduced during the period in which conditions are adapted to it, it can 

 never develop. 



In a lake, conditions are rehitively static and .-i large per cent of the 

 forms capable of developing in it at a given stage in its history, succeed in 

 reaching it, while in a pond this per cent is much smaller, Forel ('04, p. 

 408) states that the similarity of the microfauna (i. e., passive migrants) 

 of one lake to that of another is due to the 'reaction reciproque d'un lac a 



