127 
northern part. This is due to the greater precipitation during 
the winter months in the southern part, the rainfall during the 
remainder of the year being practically the same in all parts 
of the state. 
The Run-off—Eighty per cent. of the Illinois River water- 
shed lies in two principal basins, each with peculiar climatic 
and topographical conditions. These basins meet at Havana, 
and consequently the conditions in the northern one constitute 
a prominent feature in the environment of the plankton at 
that point. As the two basins he in different storm-tracks 
their rain-floods do not always coincide. The southern basin 
usually parts with its snow several days sooner in the spring, 
and more promptly, than the northern, since its latitude is 
more uniform, and, relatively, its floods are larger. The 
climatic conditions of the southern basin are manifest at Ha- 
vana principally in the backwater from floods in the lower 
river, which check the current and delay the run-off from the 
northern basin. 
The subject of fluctuations in the volume of water in the 
river is one of fundamental importance in our plankton work. 
With rising water comes a decided increase in the current and 
a rapid displacement of the sluggish waters teeming with mi- 
nute life by a turbid flood to a large extent devoid of life. 
With the overflow of the bottom-lands the subordinate 
lagoons, lakes, and marshes are for the time being obliterated 
from the landscape, and their peculiar fauna and flora mingled, 
merged, and swept away with the flood. With the decline of 
the flood, the unusual prolongation of which in the [linois 
River has been already alluded to, a great variety of conditions 
of current, temperature, depth, light, and vegetation are 
afforded, the most of which favor the development and diversi- 
fication of the plankton. ‘The large amount of silt—composed 
of a wide variety of substances, from an impalpable earthy 
material to coarse sand and the comminuted debris of vegeta- 
tion—introduced by flood water presents a most perplexing 
problem in our quantitative plankton work, Aside from the 
