344 



ANIMAL BIOLOGY 



orating mechanisms to meet particular vicissitudes, has lumped 

 all other vicissitudes into one and made a means of meeting them 

 all" — the nervous mechanism. 



That organisms respond to environmental changes, we are well 

 aware. Life itself is the result of — in fact, is — a continuous flow 

 of physico-chemical actions, interactions, and reactions with the 

 surroundings. But by the behavior of the organism we refer 

 specifically to the reactions of the organism 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 particular, from the lowest to the highest, a great 

 though gradual increase in complexity. The behavior of Amoeba 



1 2 8 



Fig. 225. — 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. (From Jennings.) 



or Paramecium is an expression of the primary attributes of pro- 

 toplasm — 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 Parame- 

 cium in its daily life under natural conditions. An individual 

 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. It tries various new positions 

 until its anterior end is directed upward, and continues in that 



