2 INTRODUCTION. 



should be borne in mind, however, that, in the case of what are 

 called " fossils " or " petrifactions," we have mineral bodies 

 which owe their existence and characters to living beings 

 which existed at former periods in the history of the earth. 

 For this reason, fossils, though composed of mineral matter, 

 can hardly be said properly to belong to the mineral kingdom. 

 On the other hand, the objects which belong to the animal 

 and vegetable kingdoms differ from those which are comprised 

 in the mineral kingdom in the following points : a. They are 

 composed of few chemical elements, of which carbon, hydro- 

 gen, oxygen, and nitrogen, are the most important ; and these 

 elements are combined to form complex organic compounds, 

 which always contain a large proportion of water, are very un- 

 stable, and are prone to spontaneous decomposition. #. They 

 are composed of diverse or heterogeneous parts, which have 

 usually more or less definite relations to one another. These 

 heterogeneous but related parts are termed " organs," and the 

 objects possessing them are said to be " organized." Some of 

 the lowest forms of animals have bodies composed of so uni- 

 form a substance that they cannot be said to be organized, as 

 they exhibit no definite organs. This exception, however, does 

 not affect the general value of this distinction, c. They are 

 always more or less definite in shape, presenting concave and 

 convex surfaces, and being bounded by curved lines, d. When 

 they increase in size, or " grow," they do so, not by the addi- 

 tion of particles from the outside, but by the reception of 

 foreign matter into their interior and its assimilation there 

 (technically called "intussusception"). e. They invariably 

 pass through certain periodic changes in a definite and dis- 

 coverable order ; these changes constituting life. They are 

 subject to the same physical and chemical laws as those which 

 govern dead matter, but the living body is the seat of some- 

 thing in virtue of which it can override the physical laws which 

 enslave mere dead matter. The living body, so long as it is 

 a living body, is the seat of energy, and can overcome the 

 primary law of the inertia of matter. It has certain relations 

 with the outer world other than the merely passive ones of 

 dead matter. However humble it may be, and even if it be 

 permanently rooted to one place, some part or other of every 

 living body possesses the power of spontaneous and inde- 

 pendent motion a power possessed by nothing that is dead. 

 In the higher animals the relations of the living body to dead 

 nature become still further complicated, and their mastery 

 over the physical forces becomes more and more pronounced, 



