ANIMALS AND THEIR ENVIRONMENTS ADAMS. 535 



water is of such a volume that, having a good current, it rushes over 

 the obstacles in its path; then a permanent brook has been evolved, 

 and a permanent rapid-water habitat has originated. 



As the erosion of the stream advances, organic debris not only 

 multiplies indigenously in the water, but it is also washed and blown 

 in, and through its decay the composition of the water is changed, 

 particularly in the amount of C0 2 present. This gas causes the 

 water to take into solution a greater amount of lime ; and at the same 

 time the agitation to which it is subjected while dashing over ob- 

 stacles or flowing over falls increases the amount of oxygen present, 

 a process further aided by the oxygen set free in it by water plants. 

 Carbonic acid, moreover, is set free by the rapids and falls. It is 

 thus very evident that the chemical processes are undergoing an 

 important development as the stream progresses, since there are 

 going on both the process of gaseous equilibrium with the air, and 

 an increase of the solids in solution. The stream is progressively 

 becoming a more favorable or enriched culture medium for organ- 

 isms. The rapidly flowing water which characterizes the brook is 

 the predominant physical feature of this environment, the stretches 

 of relatively quiet water which form the pools, between the 

 more rapidly flowing parts, anticipating the kind of conditions 

 which are destined to increase with the transformation of the brook 

 conditions into those of a creek. With the progress of development 

 in drainage a brook is progressively transformed by the processes 

 of erosion into a creek. Here the rapid-water conditions are more 

 nearly equaled by a corresponding enlargement of the pool or the 

 quieter stretches of water, where the finer sediments are deposited 

 and the animals dwelling on the surface film or in the mud and 

 sand, find suitable conditions. The falls and rapids which charac- 

 terize the brook are exceptional in the creek, but may linger where 

 the rate of change has been very slow on account of the resistance 

 of the substratum. The alternation of rapid and slower water, 

 which characterizes the creek stage, with the preponderance of the 

 relatively rapidly flowing water, is gradually transformed into that 

 of a river, where the water flows at a slower rate and rapids and 

 falls have as a rule become extinct, and where a condition of relative 

 chemical equilibrium has also been reached. Here the burden of 

 coarse debris is at a minimum, and the surface, sides, and bottom 

 of the stream have become differentiated as relatively distinct hab- 

 itats. With progressive approach toward base level all conditions 

 of the environment tend to become more stable and equalized until 

 the stream erodes to tide level, becomes brackish and finally as salt 

 as the sea itself, and reaches an equilibrium determined by the dom- 

 inant animal environment upon the earth — that of the sea. 



