i8o BEHAVIOR OF THE LOWER ORGANISMS 



made as simple as possible. All sources of stimulation save one are 

 excluded, in order that we may discover the precise effects of that one. 

 In our account of Paramecium we have seen that when more than one 

 source of stimulation is present, the behavior is determined by all the 

 existing conditions, so that often the behavior cannot be characterized as 

 a precise reaction to a definite stimulus. That this is true also for other 

 infusoria we have seen in a number of instances, particularly in our ac- 

 count of the contact reaction. It would be possible to add many other 

 examples to these, making a special chapter on "Reactions to Two or 

 More Stimuli," but this would add no new principle to what we have 

 already brought out. The general statement may be made, that to 

 account for the way an infusorian behaves at a given time, it is as a rule 

 not sufficient to take into account a single source of stimulation, but all 

 the conditions must be considered. 



We shall now look at certain features of the behavior of infusoria 

 under the conditions that are supplied by the environment, in all their 

 variety and complexity. We wish to see how the natural "wild" or- 

 ganism behaves. Our account cannot be exhaustive, for the natural 

 history of the thousands of species of infusoria remains largely to be 

 worked out. We shall merely examine certain typical features of the 

 behavior, devoting especial attention to the food reactions. 



In our chapter on the "Action System" we have seen some of the 

 chief variations in the natural behavior of infusoria. We have there seen 

 that the infusoria can be divided, according to their methods of life, into 

 three main groups : those that are attached, those that creep over sur- 

 faces, and those that swim freely. The behavior in these different groups 

 necessarily differs much. Yet, as we have seen, every possible gradation 

 exists from one group to another, and even the same individual may at 

 different periods represent each different group. The behavior is sim- 

 plest and least varied in the free-swimming organisms ; more varied in 

 those which habitually creep along a surface; most complex in those 

 which live attached. The reason for this seems to be as follows: In 

 the open water the conditions are exceedingly simple. The free-swim- 

 ming organism may escape an injurious stimulus simply by swimming 

 away. In the fixed organism, on the other hand, the conditions are more 

 complex. At any moment both the solid and the free fluid are acting 

 on the organism. For a fixed animal to obtain food and escape injurious 

 conditions, varied devices are necessary. It cannot at once solve any 

 difficulty by departing, as the free organism can. We find, then, that 

 such fixed organisms have developed varied reaction methods (see the 

 preceding chapter). 



There is much variation in the complexity of behavior even among 



