Development? 



I he essence of life is change. The essence of 

 death is inertia. All of us learn this at an 

 early age. We watch the waxing and 

 waning of the seasons and observe a reg- 

 ular succession of changes in the living 

 things about us. We watch the ant on a 

 blade of grass, the fish in a bowl, the dog 

 in our back yard, and find them in states 

 of ceaseless activity. Some are short-term 

 activities such as moving, eating, excret- 

 ing, mating; some are long-term activities 

 that become apparent only after the pas- 

 sage of weeks and months. Ultimately, 

 we recognize these changes in ourselves 

 as we grow and develop from babyhood 

 to adulthood. 



The biologist takes a more pene- 

 trating view of biological activity and 

 distinguishes three classes of alterations: 



1. Short-term physiological and 

 morphological alterations. Many of these 

 What changes are quite familiar to us. For ex- 



ample, our body temperature may vary 

 I S from hour to hour depending on whether 



we are awake or asleep, at work or at 

 rest. The color, texture, and thickness of 

 an animal pelt can vary rather signifi- 

 cantly from winter to winter in the life 

 of an animal, depending on the severity 

 of each winter. A callus may form on a 

 hard-working hand and disappear when 

 the hand's owner takes up a more seden- 

 tary occupation. Sudden anger may in- 

 crease the heart beat, divert blood from 

 intestines to muscles, and augment the 

 rate of breathing. 



As still another example, if we ex- 

 pose a population of yeast cells, which 

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