SECONDARY COMMUNITIES 17 
(24) into streams, ponds, and lakes. The effect of the industrial wastes 
differs with their character. Sewage practically destroys all the life of a 
stream or lake near the point of entrance, through the introduction of 
many poisonous substances, through the increase of carbon dioxide 
and ammonia and through the lowering of oxygen content. Nichols (25) 
states that the oxygen above the entrance of the Paris sewer into the 
Seine was 9. 23 c.c. per liter, and immediately below 1.05 c.c. per liter, a 
reduction of almost go percent. The typical swift-water fauna of Thorn 
Creek at Thornton was reduced to practically nil by the opening of 
the Chicago Heights sewage system. The common isopod (A sellus com- 
munis) was the only animal able to withstand the conditions. At a 
distance from a point of entrance of sewage the amount of plankton 
is increased by its introduction because of the nitrogen and other food 
for plants which it contains. Forbes (see 5a) reports that the amount 
of plankton near Havana in the Illinois River has doubled since the 
opening of the Drainage Canal. 
5- EQUILIBRIUM IN THE SECONDARY COMMUNITIES 
Equilibration means a restoration of balance in the numbers of 
contending organisms of the community. For instance, as has already 
been noted, the deer reached their maximum number with the correspond- 
ing destruction of the carnivores by man. This indicated that the 
primeval balance between the carnivores and the herbivores had been 
disturbed. An entirely new balance has now been established through 
the complete destruction of both the large hervibores and carnivores, 
by man. Most of our knowledge of equilibration in communities has 
resulted from the study of the secondary communities of parks and 
agricultural lands. Concerning these Forbes (26, p. 15) has said: 
There is a general consent that primeval nature, as in the uninhabited forest 
or the untilled plain, presents a settled harmony of interaction among organic 
groups which is in strong contrast with the many serious maladjustments of 
plants and animals found in countries occupied by man. [All our serious out- 
breaks of insect pests are instances of these maladjustments.] 
To man, as to nature at large, the question of adjustment is of vast impor- 
tance, since the eminently destructive species are the widely oscillating ones. 
Those insects which are well adjusted to their environments, organic and inor- 
ganic, are either harmless or inflict but moderate injury (our ordinary crickets 
and grasshoppers are examples); while those that are imperfectly adjusted, 
whose numbers are, therefore, subject to wide fluctuations, like the Colorado 
grasshopper, the chinch bug, and the army worm, are the enemies which we 
have reason to dread. Man should then especially address his efforts, first, 
