538 
midsummer decline and the autumn maximum are evident, but 
not in all cases well defined. The environmental factors com- 
mon to both, especially that of temperature, thus tend to pro- 
duce similar effects. Throughout all these changes, save in the 
low-water period of 1897, the marked contrast in the total 
population continues unbroken, a constant witness to the effect 
of the great points of difference in the two habitats, namely, 
the age of their respective waters and their relative richness in 
the immediately available elements for the support of hfe. It’ 
is only when these points of contrast in the environment van- 
ish, by reason of the slight current, as in the low water period 
of 1987, and by reason of the increase of the products of or- 
ganic decay at such times inthe water of Spoon River, that the 
plankton of the two streams approaches equality in production 
and exhibits a comparable assemblage of constitutent organ- 
isms. 
The contrast between the autumn planktons of 1896 and 
1898 and that of 1897 is not only due to the low water of this 
year, but also to frequent flushings which were caused by the 
many minor rises during the autumn of the two years first 
named. (See Plates X,, XI.) These autumnal rises are due to 
general rains, which, as a rule, affect both Spoon River and the 
main stream at about the same time. The difference in their 
effect upon the plankton of the two streams is very evident in 
the data of Table XIV. The autumnal plankton of the Illi- 
nois shows a somewhat similar population in all three seasons. 
Spoon River, on the other hand, shows a great falling off in 
numbers—from several million to a few score thousand—in 
both autumns, in which these rises are frequent. ‘The effect of 
these minor floods is thus deleterious to the plankton of the 
tributary in accordance with the general principle that they 
decrease the time for breeding the plankton in its waters. They 
also prevent the development of plankton maxima in the 
tributary by repeated removals of whatever accumulations are 
produced. Owing to the differences in their volumes and in 
the slope of their channels, these flushings are more complete 
