AiiiciiLai) Mustura of \aluijl History 



ADJUSTMENT TO EXTREME CONDITIONS 



Inhabitants of this tide pool or of the ocean shore must be tough to stand the beating 

 waves and the rushing tides. Twice daily they are exposed to the drying air and 

 then submerged again. The surrounding conditions ore of many kinds, but the or- 

 ganisms survive the variations, which are fairly regular, or periodic, and limited 

 in degree 



How Is the Balance of Nature Upset? 



Life Is Always Upsetting When a plant-animal community has reached 

 a fairly stable "climax", it contains the greatest amount of living matter that 

 the particular region can sustain. This situation is similar to that in a balanced 

 aquarium with green plants or in a "ripe" hay infusion. Such a condition of 

 balance is like the resting position of a pendulum or of a scale-beam — it is 

 easily upset, by a comparatively slight disturbance. 



The balance of nature, however, is not a state of rest. It is more like the 

 continuous swinging of the pendulum back and forth, within certain limits. 

 We might perhaps better speak of the hahndng. For the condition in a for- 

 est, for example, or in a tide-pool is one of continuous change (see illustration 

 above). And it is also one of continuous <fx-change. Materials move from 

 the air and soil into green plants, from plants to animals, and eventually back 

 to soil and air. 



The relative numbers of the different species may remain essentially the 

 same indefinitely, though they do, of course, fluctuate from day to day and 



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