24 



particularly in the amount of CO2 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 obstacles 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 devel- 

 opment as the stream progresses, since there are going on both the 

 process of gaseous equilibrium with the air, and an increase of the sol- 

 ids in solution. The stream is progressively becoming a more favor- 

 able or enriched culture medium for organisms. The rapidly flowing 

 water which characterizes the brook is the predominant physical fea- 

 ture 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 trans- 

 formation of the brook conditions into those of a creek. With the 

 progress of development in drainage a brook is progressively trans- 

 formed 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 

 characterize the brook are exceptional in the creek, but may linger 

 where the rate of change has been very slow on account of the resist- 

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

 which characterizes the creek stage, with the preponderance of the 

 relativelv rapidlv 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 chem- 

 ical 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 habitats. With 

 progressive approach toward baselevel all conditions of the environ- 

 ment 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 dominant animal environ- 

 ment upon the earth — that of the sea. 



We have now outlined the developmental sequence of wet depres- 

 sions, the lake-pond-swamp series, and the running water, the brook- 

 creek-river series, these two series including the main inland animal 

 environments in a liquid medium in a humid climate. We have yet to 

 consider the animal environments of land animals proper, those which 

 live in the gaseous medium of air. The complexity of conditions upon 



