23 



verted into streams or dry land. Let us consider the streams, particu- 

 larly those which did not develop from the lake, pond, and swamp se- 

 ries, in order to consider them in their simpler conditions of d^evelop-' 

 ment. 



The first shower on the new land surface, or the beginning of a 

 cycle, forms an extensive ramification of small streamlets, their dert^ 

 dritic branches flowing down all slopes. With the confluence of the 

 smaller branches the progressively larger trunks are formed, and with 

 their increase in volume, cutting progresses; but all traces of this 

 stream itself tend to vanish soon after the shower is over, although 

 some water may linger in pools in the deeper depressions. These con- 

 ditions form an initial stage in the development of the activity of run- 

 ning water as an animal habitat. These temporary streams are rain 

 waters intermingled with dust from the air and soil from the ground. 

 Since, viewed chemically, such waters have not existed as a liquid long 

 enough to dissolve much gaseous and solid material, they represent a 

 relatively original condition, or an initial stage in the chemical devel- 

 opment of the stream as a medium for living animals. Again and 

 again these showers are repeated, and where there is a slight variation 

 in the hardness of the substratum small pools are formed on the softer 

 materials, where erosion is more rapid. In these pools it is possible 

 for some aquatic or amphibious animals, of marked powers of disper- 

 sal, to become lodged, or even entrapped, as in the case of animals 

 which migrate up the stream during its temporary flow ; such pools, 

 in fact, may be reached even by individuals from the ground water. 



Finally these temporary streams cut down to ground-water level 

 and become permanent. Such a stream then, in addition to the fresh 

 rain-water which it receives with each shower, has a permanent supply 

 of ground water. This water, having filtered through the soil, con- 

 tains both gas, particularly COo, and minerals, and thus as a solution 

 differs much from rain water. The composition of ground water 

 varies much with the chemical differences of the substratum. Such 

 water generally contains enough substance in solution to be a favor- 

 able medium for plant growth, such as alga? — aquatic pioneers which 

 are comparable to the lichens in their invasion upon bare rock. But 

 the temporary flow of water is still dominant, and will remain so until 

 the supplv of permanent ground water is of such a volume that, hav- 

 ing a good current, it rushes over the obstacles in its path ; then a per- 

 manent brook has been evolved, and a permanent rapid-water habitat 

 has originated. 



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

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

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



