DISTRIBUTION OF SPECIES. (55 



spina, Diaptomus sicilis, Cyclops pulchellus. Peculiar to the 

 large deep lakes are Limnocalamis macrurus, Mysis, and Poitr 

 toporeia, all being forms favored by trie deep water conditions 

 of the lakes of this class. 



Diaptomus Ashlandi should be added to the forms found only 

 in large deep lakes, although it is not common to all large deep 

 lakes, but is peculiar to the Great Lakes. 



Daphnia pulicaria may, I think, be considered a distinctive 

 form of the small lakes, although sometimes found in lakes that 

 perhaps would not fall in this class. 



The following may be considered as distinctively shallow 

 lake forms : Clatlirocystis, Oscillaria, Diaptomus oregonensis, 

 and, perhaps, Eurycercus. The cladocera are much more abun- 

 dant both in numbers and in species in the shallow lakes than 

 in the deep lakes, but the forms are not peculiar, as they are 

 generally littoral forms which have become limnetic in the lakes 

 where the littoral and limnetic environments are very similar. 



COMPARISON OF PLANKTON OF SUCCESSIVE YEARS. 



It is evident that the plankton varies greatly in one year as 

 compared with another, and that the variation has no immedi- 

 ate connection, in many cases, with differences in annual tem- 

 perature. I take it that the variations may be explained by the 

 relations of the organisms to each other. 



It has been a favorite notion of many biologists, in which I 

 have shared, that the balance of life in a lake undisturbed by 

 man is maintained with a good degree of exactness ; that the 

 animal production is based, in amount, on the plant production, 

 and that the number of predaceous animals is conditioned on 

 the numbers of the animals that serve as their prey. It would 

 follow, of course, that in a lake where there were great possi- 

 bilities in the way of plant growth, there would be a correspond- 

 ingly large number of animals. From this has arisen the com- 

 mon inference that it is the shallow lakes, in which the amount 

 of vegetation is greatest, that are the best for the production of 



