PHYSIOLOGICAL ANIMAL GEOGRAPHY 597 
Many of the factors and conditions employed in such experiments 
are of such a nature that the animal never or rarely encounters 
them in its regular normal life. Other experiments are, however, 
attempts to keep the environment normal, except for one factor 
(Jennings, 06, p. 180). These have demonstrated that in ordi- 
nary reactions an animal responds to the action of a single stimu- 
lus. Certain general laws govern the reaction of animals to 
different intensities of the same stimulus. 
a. Laws governing the reactions of animals. The laws govern- 
ing the stimulation of animals in the experiments of the laboratory 
are familiar subjects in the textbooks of physiology (Verworn- 
Lee 99). With respect to a given factor used in the experiment, 
it has been found that there is a range of conditions within which 
the activities of the animal proceed without marked stimulative 
features. These are called optimal conditions. Take, for exam- 
ple, temperature. There is in most animals which have been 
subjected to experimentation with temperature, a range of several 
degrees in which the animal is not markedly stimulated(optimum). 
‘As the temperature is raised or lowered from such a condition, 
the animal is stimulated. If the temperature be continuously 
raised, a point is reached at which the animal dies. The tempera- 
ture condition just before death occurs is called the maximum. 
The lowering of temperature produces results comparable in a 
general way to those of high temperature. The condition just 
before the death point is reached is called the minimum. With 
various limitations, unimportant in this connection, the same is 
true with respect to each of the various factors which an animal 
encounters in nature. Which factor determines the limitations 
of occurrence of an animal on the earth’s surface? The answer 
to this is suggested in Liebig’s Law of Minimum. 
b. Law of minimum.  Liebig’s law of minimum is summarized 
by Johnstone (’09, p. 234): 
A plant requires a certain number of food stuffs if it is to continue to 
live and grow. Each of these food substances must be present in a cer- 
tain proportion. If it is absent the plant will die; if present in a minimal 
proportion the growth will also be minimal. This is true no matter how 
abundant the other food stuffs may be. The growth is then dependent 
upon the amount of food stuff present in minimal quantity. 
