24 Introduction 



to the other. The resulting organism usually has a definite recog- 

 nizable form. 



Irritability. — The migratory movements of birds in the spring and 

 fall, the flight of the moth toward a light — these are but a few examples 

 of the response of organisms to changes in their environment. Such 

 irritability is not found in nonliv^ing systems. Any change in the en- 

 vironment which affects an organism is known as a stimulus. Stimuli 

 may be either of an external or internal origin. Changes in tempera- 

 ture, pressure, light, wind, etc., would all constitute stimuli from the 

 external environment. Sensations such as hunger and thirst are stimuli 

 from the internal environment of the organism. 



Organization. — Every living organism has a definite form; so 

 constant is this form that it is possible to recognize kinds of animals 

 without difficulty. This form is due to a definite organization of the 

 various individual parts of the animal in relation to one another. This 

 organization is not confined to the over-all structure ; it is found 

 through every individual part down to the smallest units. 



The organization of living materials may extend beyond the indi- 

 vidual itself to complex social systems such as those illustrated by the 

 beehive or even that of human society. 



Reproduction. — The ability to reproduce themselves is a unique 

 characteristic of living things. In general, the offspring resemble the 

 parents, but there are exceptions. In those cases where the offspring 

 bear but slight resemblance to the adult, the new individual undergoes 

 a series of orderly developmental changes and gradually takes on the 

 appearance of the parent. 



Metabolism. — All these dynamic processes that distinguish living 

 things from nonliving ones are made possible by the ability of the or- 

 ganism to take in materials, convert them, and reassemble the parts 

 which are then utilized for growth, repair, reproduction, and energy. 

 This vital process is called metabolism. 



There are two aspects to metabolism. The building up of new 

 parts is known as anabolism. The tearing down of these parts is 

 known as catabolism. In a healthy organism one process balances the 

 other. When catabolism overbalances anabolism, degeneration takes 

 place. 



