340 FOUNDATIONS OF BIOLOGY 



the organism we refer specifically to the reactions of the or- 

 ganism as a unit, rather than to the internal processes in the 

 economy of its life. And surveyed from a broad viewpoint, 

 there is discernible in the behavior of animals, just as in their 

 structure in general and in their nervous system in particu- 

 lar, from the lowest to the highest, a gradual increase in the 

 complexity of behavior. The behavior of Amoeba or Para- 

 mecium is an expression of the primary attributes of proto- 



FIG. 183. Diagram to illustrate the avoiding reaction of Paramecium. A, a 

 solid object or other source of stimulation. 1-6, successive positions taken by the 

 animal. The rotation on its long axis is not indicated. See Fig. 184. (After Jennings.) 



plasm irritability, conductivity, and contractility. So is the 

 behavior of Hydra and Earthworm, in which special cells 

 constitute a definite coordinating, or nervous, system. And 

 so is the complex behavior of the higher animals, including 

 Man, with their elaborate series of sense organs and highly 

 developed sensorium, or brain. 



"Let us now try to form a picture of the behavior of Par- 

 amecium in its daily life under natural conditions. An indi- 

 vidual is swimming freely in a pool, parallel with the surface 

 and some distance below it. No other stimulus acting, it 

 begins to respond to the changes in distribution of its internal 

 contents due to the fact that it is not in line with gravity. 



