449 
SEASONAL COMPARISONS. 
The area herein discussed is typical of many in the middle west, 
especially in those states bordering the southern part of Lake Michi- 
gan. It is typical also of those areas in which the volume of water 
fluctuates through the seasons of the year as well as in the same season 
in different years. In spring, there is an abundance of water in all 
the streams and ponds, and every depression in the woods (as shown 
in Plate XVI.) is filled with water and supports some kind of animal 
life. Spring conditions are shown in plates XVII., XVI.,2, XX., 
XXIII., and XVIII.,2. In the fall the water evaporates, either en- 
tirely or to such an extent as to leave only small pools here and 
there. This condition is shown in plates XVII.,1, XVIII.,1, and 
XXIV.,1, which should be compared with the photographs of the 
same habitats in spring. 
The difference in one year as compared with another may be seen 
by comparing plates XVII.,1,and XXIV.,1, which were taken Sep- 
tember 5, 1908, with plates XVI. and XXIII., of the same habitats, 
taken September 10, 1909. It will be recalled that the year 1908 
was much drier than 1909, this difference in precipitation causing a 
marked effect on the summer-dry ponds of this area. The year 
1909 has, therefore, been much the more favorable year for inverte- 
brate life in these ponds and streams, owing to the less rigorous 
conditions of the environment. The effect has also been notably 
different on the vegetation, which was much more luxuriant in the 
fall of 1909 than at the same period in 1908. 
The five areas just described break up into a number of more 
or less distinct stations, the biota of which differ more or less. These 
stations will next be taken up in detail. Their positions may be 
ascertained by consulting the map. 
A. SKOKIE MARSH. (Stations I ann II.) 
STATION I. 
(Plate VI.) 
Skokie stream and tributary ditches. This stream was once a 
tributary of the East Branch, but it now ceases to exist at a point 
southwest of Glencoe. It is an intermittent stream, in the spring 
