QQ PLANKTON OF WINNEBAGO AND UREEN LAKE 8. 



fish. It has also been inferred, — I have made the statement 

 myself, — that inasmuch as in a state of nature there must he 

 brought about an equilibrium between plant and animal growth, 

 it is possible that man in his work of fish planting may disturb 

 this equilibrium so 1 as to interfere with the highest productive- 

 ness of a lake ; for it seems apparent that if an equilibrium be- 

 tween plants and animals is once established, the introduction 

 of any considerable number of either plant-eating or predaceous 

 animals would bring in an element that would disturb nature's 

 balance. The! result of my work on these lakes leads me to 

 think that this idea is very far from correct. Of course, it is 

 true that in the long run a balance will be reached, but there 

 may be wide swings of the: pendulum on either side of this con- 

 dition of equilibrium. It is evident that the maxima, under 

 similar conditions of light and temperature, are not reached at 

 the same time in different years, nor is the annual rotation of 

 forms the same in different years. Some animals, according 

 to Birge', seem to have biennial periodicity, but others, in which 

 this is certainly not the case, appear in great numbers in some 

 years and not in others. There is great variability in the an- 

 nual records of a lake. It is apparent that certain forms, un- 

 der favorable conditions, — conditions which it is frequently im- 

 possible to define, — seem to get such a start in the competition 

 of life that they are produced in great numbers in some years, 

 while in others they may hardly appear. Some animals, if the 

 expression may be used, seem at times to get a momentum which 

 carries them on to i great production. When one stops to think 

 of it this is no more than would be expected. Just as weeds 

 overrun a farm, so certain forms of aquatic vegetation may, 

 overtop everything else in a given year's production. The con- 

 dition of a lake cannot be compared with that of a forest un- 

 touched by man, for in the forest the vegetation is largely per- 

 ennial, while in the lake the vegetation disappears each year, 

 and in the annual reappearance it is conceivable that some form, 

 from some little changes in the environment, may get a start 

 that will shut off other forms. Thus, in spite of the fact that 



