112 evolution: the ages and tomorrow 



in the simplest cell. An amoeba moves about by bulging out 

 its shapeless body into protruding "false feet" in whatever 

 direction it wishes to go. Other cells show restless, flowing 

 movements. In plant cells there is an endless circulation of 

 protoplasm within the tiny cellulose boxes. It has been 

 shown that the movements and contractions of amoeba have 

 many properties in common with the action of a muscle cell 

 in higher forms, including man. The two are alike in the 

 basic chemistry of the activity, and it is apparent that the 

 muscle cell in man has merely exploited the primitive prop- 

 erty of contractility, or the power of drawing muscle fibers 

 into more compact form, to the fullest extent. In higher 

 animals, muscle cells of several types are responsible for the 

 movements of the organism. Some are under control of the 

 will and are called voluntary muscles; some are controlled by 

 lower centers of the brain and are called involuntarv; and 

 there is a special type concerned in the movements of the 

 heart. These are very special cells with amazingly high effi- 

 ciency in contractility. Consider the heart muscle of man 

 set in its life-giving rhythm of contraction throughout the 

 years; or the tremendously powerful muscles which move 

 the wings of birds. In nature there are many intermediate 

 kinds of muscles, and one can see an evolutionary history 

 here starting with the unspecialized contractility of primi- 

 tive unicellular organism and ending in the insect and man. 

 Movement is not the only way in which an organism re- 

 sponds to stimuh. Glands of various kinds respond, and se- 

 cretions are released which act to change the chemistry of 

 the body in digestive juices, hormones, perspiration, etc. 

 Nature has used this method to produce marked changes in 

 the behavior of animals and to protect them, as in the slime 

 and poison glands of some amphibia and of snakes. Even a 

 luminous secretion was discovered by nature and is used in 

 nocturnal organisms of many kinds, usually to aid them in 

 finding mates. Sometimes there are bizarre accessories, as in 

 some deep sea fish which have a reflector and lens to con- 



