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438 
surrounding trees cover the bottom so that a very distinct but uniform 
condition is developed. Besides their 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 with an area of about one acre 
was formed. ‘This is the typical phenomenon in the formation of solution 
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 aquatic habitat is 
formed with no aquatic fauna. The first forms introduced into a pond at 
this stage have 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 
their fundamental difference in size) is the paucity of species in the former 
when 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 Pléner See, Zacharias 
(93) reports 20 species and varieties of cladocera, one ostracoda and 10 gas- 
tropoda. Burehardt (00) observes that the plankton of the Alpenacher 
See contains fewer species than the Vierwaldstiidtersee. Dr. W. Halbfass 
collected from a number of lakes in uorthern Germany and sent the ma- 
terial to Zacharias for examination. ‘The lists show only one Cladoceran 
from Dolgensee bei Neustettin, . very small body of water, while Wilmsee, 
a much larger lakelet, contained six. The faunal 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 particular period, and to the fact that the 
pond after its formation, changes very rapidly, thus making it suited for 
a particular species 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 2 lake, conditions are relatively static and a 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 - (O04, p. 
408) states that the similarity of the microfauna (i. e., passive migrants) 
of one lake to that of ancther is due to the “reaction reciproque d’un lae a 
