Living things include both the visible world of animals and plants as well as the invisible world of bacteria and viruses. On a basic level, we can say that life is ordered.  Organisms have an enormously complex organization. Life can also “work”. Living creatures can take in energy from the environment. This energy, in the form of food, is changed to maintain metabolic processes and for survival. Life grows and develops. This means more than just getting larger in size. Living organisms also have the ability to rebuild and repair themselves when injured.  Life can reproduce. Life can only come from other living creatures. Life can respond.  Think about the last time you accidentally stubbed your toe. Almost instantly, you moved back in pain. Finally, life can adapt and respond to the demands placed on it by the environment. There are three basic types of adaptations that can occur in higher organisms.

            Reversible changes occur as a response to changes in the environment. Let's say you live near sea level and you travel to a mountainous area. You may begin to experience difficulty breathing and an increase in heart rate as a result of the change in height. These signs of sickness go away when you go back down to sea level.

            Body- related changes happen as a result of prolonged changes in the environment. Using the previous example, if you were to stay in the mountainous area for a long time, you would notice that your heart rate would begin to slow down and you would begin to breath normally. These changes are also reversible. Genotypic changes (caused by genetic change) take place within the genetic make up of the organism and are not reversible. An example would be the development of resistance to bug-killing chemicals by insects and spiders.

What does the word “Reversible” in the passage mean?