156 



ANALYSIS OF THE ENVIRONMENT 



like alpine meadows or of the cascade type 

 of violently descending waters. This general 

 subject is discussed again in the chapter 

 on Succession (p. 572). 



Streams frequently show a transverse as 

 well as a longitudinal asymmetry. This is 

 especially noticeable in eroding streams 

 where one bank, the outer bank in a bend, 

 is being under-cut and material is being 

 deposited on the opposite margin. 



Adaptations to Stream Life 



Animals that live in rapid currents usu- 

 ally are adapted by structure or habit, or 



hand, propel, and on the other resist its 

 passage down the oviduct. The stream- 

 lining of the bodies of birds and fishes is 

 brought about, not by direct action of the 

 environment, but by evolutionary processes. 

 Evolution has also aflFected the form of the 

 eggs of many species of birds. 



In the hen's egg, the foremost part be- 

 comes the broad end, and the cross section 

 of greatest width lies well forward as the 

 plastic egg is moved down the oviduct; the 

 posterior part tapers over a longer distance. 

 The same relations are further developed in 

 the streamUned bodies of fishes, although 



Fig. 30. Diagrammatic arrangement of streams entering Lake Michigan north of Chicago. The 

 numbers show progressive stages in succession. ( Redrawn from Shelford. ) 



both, to maintaining their position against 

 the sweep of the current. Some of the struc- 

 tvural devices include suckers of various 

 sorts, attachment threads, and gripping ap- 

 pendages and surfaces. Adaptations in habit 

 include the darting movement of certain 

 fishes that is often combined with the tend- 

 ency to take shelter below or under pro- 

 jecting stones or among vegetation when 

 that is present. 



A more universal adaptation to life in a 

 current or to active movement through 

 water or air is the development of a stream- 

 lined form. A form is said to be stream- 

 lined when air or water flows around it 

 so smoothly that resistance is reduced to a 

 minimum. A hen's egg is automatically 

 streamhned by the direct action of the 

 combination of pressures that, on the one 



with them the motive force is internal and 

 only the environmental resistance is fur- 

 nished by the surrounding water. The 

 rounded head parts the water with a min- 

 imum of eddy formation; the parted me- 

 dium closes in on the elongated, gently slop- 

 ing posterior body and so tends to restore 

 some of the energy expended in making the 

 original separation of the water. The grad- 

 ual slope of the posterior body also avoids 

 the retarding water eddies that result from 

 flow past a more angular figure. 



Study of the resistance of models to 

 movement through water dates back at least 

 to 1775. The celebrated physicist, Clerk 

 Maxwell, explained certain of the laws in 

 1854. Clemens (1917) used models of 

 equal weight, made in each case by using 

 the same wax, and found the pull needed 



