302 AN INTRODUCTION TO ZOOLOGY 



lution, both directly, by securing the well-being of the species 

 while diminishing the waste of energy, and indirectly by favoring 

 the growth of intelligence. . . . Therefore combine practice 

 mutual aid. That is the surest means of giving to each and to 

 all the greatest safety, the best guarantee of existence and prog- 

 ress bodily, intellectual, and moral. That is what natui 

 teaches us." 



3. REFLEXES, INSTINCTS, AND THE ANIMAL MIND 

 a. Reflexes 



In the Metazoa the reflex, carried on either consciously or ui 

 consciously, is considered the physiological unit of nervous ac- 

 tivity. It requires in its simplest form a sensory neuron, th( 

 receptor, a motor neuron, the adjuster, and the organ stimulal 

 to activity, the effector (see p. 179). The stimulus passes froi 

 the receptor to the adjuster and is reflected to the effector. Tl 

 apparent reflection suggested the term " reflex." 



The reflex, according to the above idea, is operative only in tl 

 case of animals with nervous systems. If we would include 

 certain lower Metazoa and the Protozoa, our definition must 

 changed so as to take into account the action, and not tl 

 mechanism that is responsible for the action. According to this 

 view, any simple response to a stimulus is a reflex. The avoiding 

 reaction of Paramecium (Fig. 33, p. 74) is known as a reflex, an< 

 the activities of Paramecia and many other lower organisms ai 

 supposed by some to be a series of reflexes to external stimuli. 

 But if examined carefully, the reactions of lower organisms ai 

 not as simple as they at first appear. For example, the avoidii 

 reaction of Paramecium is not always exactly the same, 

 the animal be taken as a center about which a sphere is described, 

 with a radius several times the length of the body, then as a re 

 suit of the avoiding reaction the animal may traverse the periph- 

 eral surface of this sphere at any point, moving at the time either 

 backward or forward. Paramecium, in spite of its curious limi- 





