ACTIVITIES COMMON TO ALL ANIMALS. 25 
Plants and Animals. The differences between 
higher animals and plants are so obvious that we need 
never mistake one for the other; but, as we shall see, 
the differences grow less and less as we consider simpler 
and simpler organisms. Among the simplest living 
bodies the processes of life go on; but we do not know 
whether to call the living things themselves plants or 
animals. 
Definitions. The science which treats of living 
matter is Biology. The branch of Biology treating of 
plants is Botany, and the branch treating of animals is 
Zodlogy. 
The study of the form, structure, and position of the 
parts of a plant or an animal is Anatomy. Minute 
Anatomy studied with the microscope is Histology.’ 
The study of the functions or uses of all parts of an 
organism is Physiology. 
Activities of our own Bodies. We may learn what 
the most important activities of animals are by con- 
sidering the chief activities of our own bodies. While 
our ability to move readily from place to place, and to 
perform the many muscular acts of daily life, is doubt- 
less the most noticeable sort of activity which we share 
with other animals, it is not the most fundamental. 
Back of all movement there must be a source of power. 
In these days, men make machines which seem almost 
alive. In these some sort of power which we can 
understand causes all the movements. Springs and 
weights move clocks, steam-pressure turns the wheels 
of factories, and electric currents move our street cars. 
Our Bodies Chemical Engines. Sources of power 
can usually be traced back to heat. The movements 
of a steam-engine are due to energy set free by the 
burning of coal. The burning of coal is a chemical 
activity. The heat is caused by the union of two 
elements, carbon and oxygen. It has been found that 
the movements of living things are also due very 
largely to chemical action. Just as coal burns or 
