RELATIONS OF THE THREE KINGDOMS. 7 



stomach. All animals therefore must have a stomach. 

 In the very lowest animals, however, this organ is ex- 

 temporized for use when wanted. A single cell, an al- 

 most microscopic spherule of gelatinous protoplasm, 

 meets its food, flows around it, takes it in, and di- 

 gests it (Fig. 156, p. 240). 



Again, the food of plants is everywhere present. In 

 the form of solution it bathes the roots, in the form of 

 gases it bathes the surface of the leaves. The food- 

 taking is passive. Animals seek their food, and usually, 

 but not always, move about to gather it. This, again, re- 

 quires a reservoir in which to keep it while it is being 

 prepared for absorption. Both the kind of food and the 

 mode of taking it require a stomach. All these four — 

 viz., kind of food, the possession of a stomach, power of 

 voluntary motion, and the seeking of food or desire — are 

 closely connected with one another ; and the kind of 

 food — i. e., organic matter — is the basis of all. For this 

 necessitates appetite, therefore seeking of food, and this 

 locomotion and a reservoir to store ; therefore all are 

 characteristic of animals. 



4. Waste and Supply. — Continual internal change, 

 as already seen, is coextensive with life. But this internal 

 change is far more rapid in animals than in plants. In 

 plants, supply is always in excess of waste, and therefore 

 plants grow as long as they live. In animals, on the 

 contrary, in early life supply is in excess of waste, in 

 maturity they balance and there is no growth, in age 

 waste is in excess. Again, as we shall see hereafter, the 

 whole of animal force is derived from waste, while in 

 plants only a small part is thus derived, the rest being 

 derived from sunlight. 



We have now delimited our field of study from other 

 kingdoms of Nature — it is animals ; and our science from 

 other departments of science — it is zoology. But the 



