Factors Determining the Annual Distribution. 365 



Competition. 



The connection between the number of a species and the com- 

 petition to which it is exposed from the other limnetic Crus- 

 tacea is a subject on which little can be said, yet indications of 

 iihe effect of competition are not wanting in my observations, 

 and it may be worth while to point out some of them. The de- 

 tails fo heve trtical distribution of the Crustacea show that 

 while the number of individuals present in the upper strata of 

 the water may vary considerably from year to year, neverthe- 

 less the number does not rise beyond a certain maximum during 

 the season, and when this maximum has once been reached the 

 number remains singularly constant. We cannot, therefore, 

 avoid the conclusion that there is a certain number of Crustacea 

 which the water can support, and that this number cannot be 

 greatly exceeded. If this is the case, the numbers of one species 

 must exert an influence, more or less unfavorable, on the number 

 of the other forms present. 



In each of the summers during which I have studied the 

 Crustacea, one form predominated in the plankton, and in 

 each year the species, was different. In 1894 Diaptomus was 

 more numerous than all the other Crustacea put together. In 

 1895 Daphnia hyalina was the predominant species in number, 

 and still more in bulk, as its individuals are so much larger 

 iihan the other species. In 1896 Cyclops was almost equally 

 predominant, although at times Daphnia was nearly or quite 

 as important. My explanation of these facts is that when a 

 species secures possession of the water it is difficult for another 

 ■species to oust it so long as its reproductive power continues. 

 The causes which give an opportunity to any given species thus 

 to occupy the water are still largely unknown, or conjectural. 

 It may be said, however, that as the species become successively 

 predominant, a form whose reproductive period is at hand at 

 tlie time of the decline of a dominant form will^be able to occupy 

 the vacant space for a time. 



An instance in which the numbers of a species seem to 

 have been affected by competition is afforded by Daphnia retro- 

 curva. In August, 1895 and 1896, the number of this species 



