O 



78 AN INTRODUCTION TO ZOOLOGY 



interesting phenomena might be cited, but the entire subject is 

 too complex for brief discussion. 



Frequently Paramecium may be stimulated in more than one 

 way at the same time. For example, a specimen which is in 

 contact with a solid, is acted upon by gravity and may be acted 

 upon by chemicals, heat, currents of water, and other stimuli 

 (Fig. 38). It has been found that gravity always gives way to 



other stimuli, and that if more 

 than one other factor is at work 

 the one first in the field exerts 

 the greater influence. 



Both the spontaneous activi- 

 ties and reactions due to ex- 

 ternal stimuli are due to changes 

 FIG. 38. Paramecia stimulated by a j n the internal condition of the 

 chemical and by contact at the . , ,-,, . 7 . 7 . , 



/ , , , animal. Ine physiological con- 

 same time. They have formed r ^ 6 



a ring about a bubble of CO 2 dition of Paramecium, therefore, 



and have then come to rest determines the character of its 



against the glass supporting response. This physiological 



rods, forming two dense groups. gtate j s a dynamic condition, 



changing continually with the 



processes of metabolism going on within the living substance of 

 the animal. Thus one physiological state resolves itself into 

 another; this " becomes easier and more rapid after it has taken 

 place a number of times " (62, p. 291), giving us grounds for the 

 belief that stimuli and reactions have a distinct effect upon suc- 

 ceeding responses. 



" We may sum up the external factors that produce or deter- 

 mine reactions as follows: (i) The organism may react to a 

 change, even though neither beneficial nor injurious. (2) Any- 

 thing that tends to interfere with the normal current of life 

 activities produces reactions of a certain sort (' negative ') 



(3) Any change that tends to restore or favor the normal life 

 processes may produce reactions of a different sort (' positive ') 



(4) Changes that in themselves neither interfere with nor assist 



