Factors Determining the Annual Distribution. 
865 
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 
the 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 J 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 
-than 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 
the time of the decline of a dominant form wilhbe 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 
