534 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1917. 



developmental sequence centering about the process of the degrada- 

 tion of the land by the agency of running water. This is therefore a 

 classification of environments, not on the basis of the product, as it 

 might appear from calling it a depression or standing-water series, 

 but upon the basis of the activity or processes of the dominant agent. 



We will assume that all the lakes, ponds, and swamps, due to the 

 original relief of the land, become drained and constructively con- 

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

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

 series, in order to consider them in their simpler conditions of 

 development. 



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

 cycle, forms an extensive ramification of small streamlets, their den- 

 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, al- 

 though some water may linger in pools in the deeper depressions. 

 These conditions form an initial stage in the development of the 

 activity of running 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 development 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 dispersal, 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 the 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 perma- 

 nent supply of ground water. This water, having filtered through 

 the soil, contains both gas, particularly C0 2 , 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 sub- 

 stratum. Such water generally contains enough substance in solu- 

 tion to be a favorable medium for plant growth, such as algae — 

 aquatic pioneers which are comparable to the lichens in their in- 

 vasion upon bare rock. But the temporary flow of water is still 

 dominant, and will remain so until the supply of permanent ground 



