ANIMALS AND THEIR ENVIRONMENTS ADAMS. 533 



der different conditions ; this is the key to individuality. Thus streams 

 viewed as stages in the progressive transformation of a liquid me- 

 dium for life, may be formed in many diverse ways, and for this 

 reason the general principles of the method of genesis may be ex- 

 pressed most simply in an ideal case. Genetic series are unending^ 

 they extend into the past and will continue in the future. The point 

 of departure for study must therefore be arbitrarily chosen, and the 

 more nearly a natural basis can be approximated the simpler its ap- 

 plication becomes. For this reason a cycle will be followed here 

 which begins with a condition of stress^ advances through the process 

 of adjustment to strain^ and reaches a condition of relative equilib- 

 rium. The starting point in such a cycle we will consider as the 

 original conditions, and the later activities as the derived ones. The 

 original conditions we will assume to be an uplifted undulating plain, 

 composed of relatively homogeneous materials, in a humid climate, 

 and covered by a varied vegetation, including trees. The elevated 

 condition of the land produces a condition of unstable equilibrium 

 or stress for the rain falling upon its surface ; and, furthermore, the 

 vegetation will tend to spread over the entire surface and thus exert 

 a certain pressure also. These original conditions, are, therefore, 

 unstable and destined to change, and mutually to influence and 

 regulate one another. 



If we now imagine the rain " turned on " under such conditions, 

 what are the main processes which will operate ? The rain falling in 

 a depression will be supplemented by that which drains from the ele- 

 vations; thus, through the agency of running water a standing- water 

 habitat will have its origin. With this concentration of water will 

 come also a burden of debris from the upland; and in this way the 

 " constructive " and " destructive " processes will begin at the same 

 time. Plants will invade such a depression and add their remains. 

 Some of the depressions will overflow and the outflowing streams will 

 cut down the outlet to progressively lower levels and ultimately drain 

 the basin. On the other hand, inwash and organic debris may to- 

 gether accumulate at such a rate as to raise the level of the basin 

 above ground water and thus transform the conditions to that of land. 

 The progressive stages of the process of degradation thus favor the 

 transformation of the depression and a progressive formation of 

 lakes, which are converted into ponds and swamps, and ultimately, 

 with drainage, to dry land. For depressions we thus get a genetic 

 series which we may call the lake, pond, and swamp series. This 

 does not classify the depression series according to size, depth, char- 

 acter of water, etc., as in an artificial classification, but in the order 

 of their development or genesis through the agency of running water. 

 Accompanying this sequence there are of course changes in size, 

 depth, etc., but these are subordinated in the classification to the 

 65133°— SM 1917 35 



