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 10° 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 20°-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 
