6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



the two living kingdoms to the mineral kingdom aie the 

 converse of one another: plants making organic matter 

 from minerals, and animals destroying organic matter 

 and returning it again to the mineral kingdom. Limit- 

 ing our view now to one of these mineral plant-foods, 

 C0 8 : plants decompose C0 2 , returning the oxygen to 

 the air and retaining the carbon to make with other ele- 

 ments organic matter; animals contrarily take carbon 

 from plants in the form of organic matter of food, and 

 oxygen from the air in respiration, combine these, and 

 restore them to the air as C0 2 . Thus there is a con- 

 tinual circulation of carbon and oxygen between these 

 three kingdoms, as shown in the diagram, Fig. 2. Thus 

 the plant kingdom is a necessary intermediary between 

 the mineral and the animal kingdoms. 



This is probably the best and most philosophical dis- 

 tinction between the two kingdoms ; but even in this 

 there is a gradation as we go down the scale of life. In 

 any case we want an easier and more practical test. We 

 find this in 



3. The Possession of a Stomach. — Animals have 

 stomachs, plants have not. This is really a philosoph- 

 ical distinction, because it is closely connected with the 

 nature of the food. The food of plants is mineral. 

 This mineral food exists in solution in water, or else in a 

 gaseous state in the atmosphere. It is therefore already 

 in condition to be at once absorbed without further 

 preparation. It is thus absorbed by the surface of the 

 roots and of the leaves — external absorption. But the 

 food of animals, being organic matter, is usually in a 

 more or less solid condition, and can not be absorbed 

 until it is dissolved; and this requires time. Therefore 

 animals must have a' reservoir in which the food is 

 stored until it is reduced to a liquid condition fit for 

 absorption — internal absorption. This reservoir is the 



