322 BEHAVIOR OF THE LOWER ORGANISMS 



soon be killed, while the Paramecia would not suffer. The latter would, 

 therefore, be selected, as compared with the former. But there- exist 

 variations of reaction even among individuals of the same species. 

 Some specimens of Bursaria when stimulated by heat show a greater 

 inclination to swim freely, revolving on the long axis, than do the ma- 

 jority, that sink quickly to the bottom and cease to revolve. The former 

 are saved from the heat, while the latter are killed. In time there might 

 thus be developed a race of Bursarias that were as well protected by their 

 behavior from the action of heat as are Paramecia. 



What are the characteristics that would be preserved by natural 

 selection? First it seems clear that under usual conditions the regula- 

 tive power would tend to be preserved. So long as the environment is 

 a changing one, those individuals that can alter their behavior to lit the 

 new conditions would live, while any that cannot do so will be killed, 

 so that any variation in the direction of less regulative power will be cut 

 off. But under quite uniform conditions there might be no advantage 

 in this regulative power, and no selection based upon it. 



Second, those variations will be preserved that are in line with the 

 general tendency of the behavior. In other words, those variations will 

 persist that tend in the same direction as the adaptation of the individuals, 

 due to selection of overproduced movements and the law of the resolu- 

 tion of physiological states. This will be made clear by an illustration. 



Most ciliate infusoria may swim freely through the water, may creep 

 along surfaces, may exude mucus to form a cyst, and may burrow about 

 in the debris at the bottom of the water. Some show one habit in a more 

 marked way, others another. Let us suppose a ciliate infusorian with a 

 cylindrical body covered uniformly with cilia, that may behave in all 

 these ways. It responds to stimulation by trial of the different reactions 

 which it has at command, continuing, in accordance with the principle 

 of the resolution of physiological states, that reaction which proves 

 successful. Suppose that a number of the individuals come thus to 

 react habitually in the first of the four ways mentioned above, others in 

 the second, others in the third, and still others in the fourth. All these 

 different methods have advantages for meeting unfavorable conditions, 

 and all are found as a prevailing reaction in different ciliates. 



We have then four groups of ciliate organisms, all alike structurally, 

 but with different habits. How will natural selection act on these? 



(i) In the first group, that swim freely through the water, like Para- 

 mecium, all variations that favor quickness of reaction, rapidity of move- 

 ment, and precision of direction will be advantageous, and the indi- 

 viduals possessing them will tend to be selected. Specimens with body 

 ill-shaped for rapid movement, with cilia weak or unequally distributed, 



