300 ECOLOGY 



animal would rarely or never encounter them in its normal life. Other 

 experiments are attempts to keep the environment normal, except for 

 one factor (44, p. 180). These have demonstrated that animals are 

 capable of responding to the action of a single stimulus. 



A typical experiment to demonstrate this would consist in preparing 

 two long receptacles in such a way that one is the normal environment 

 of the animals in all respects and the other in all respects except for 

 one factor, as, for example, temperature. The temperature conditions 

 of the latter might be as follows: temperature at one end io° C, at the 

 other 35 C, with a gradient between. If then 100 animals are placed 

 in each of the receptacles, those placed in one end of the gradient will 

 soon show signs of stimulation and will move about until they come 

 near the center of the pan where the temperature is 2o°-25°. If, after 

 sufficient time has elapsed for the experimental animals to take up this 

 position, the control animals have remained equally distributed, the 

 experiment will show that the animals have responded to temperature 

 alone. 



Certain general laws govern the reaction of animals to different 

 intensity of the same stimulus. Take, for example, temperature. 

 There is in most animals which have been subjected to experimentation 

 with temperature a range of several degrees within which the activities 

 of the animal proceed without marked stimulative features, as is sug- 

 gested by the experiment outlined above. Conditions within this 

 range of several degrees are called the optimum. As the temperature 

 is raised or lowered from such a condition, the animal is stimulated. 

 If the temperature is continuously raised, a point is reached at which 

 the animal dies. The temperature condition just before death occurs 

 is called the maximum (35). The lowering of temperature produces 

 comparable results. 



2. EXPERIMENTAL STUDIES OF HABITAT SELECTION 



Animals select their habitats, and distribution is the result of this 

 selection. To decide whether or not one factor can determine distri- 

 bution, experiments, of which the following is a typical example, have 

 been performed. 



a) Methods of experimentation. — Do animals select their breeding- 

 places ? To answer this question, tiger-beetles were selected as material 

 and adults were placed in cages containing soil of several kinds. Each 

 kind was so arranged into steep and level parts, that about one square 

 foot of each type was exposed. The adults placed in the cage were 



