2 INTRODUCTION. 



" 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 mattsr, cannot 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, hydi'ogen, 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 unstable, and are prone to spontaneous 

 decomposition, b. They are composed of diverse or heterogeneous 

 parts, which have usually more or less definite relations to one an- 

 other. These heterogeneous but related parts are termed "organs," 

 and the objects possessing them are said to be "organised." Some 

 of the lowest forms of animals have bodies composed of so uniform a 

 substance that they cannot be said to be oi-ganised, 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 addition of particles from the outside, but by 

 the reception of foreign matter into their interior and its assimila- 

 tion there (technically called "intussusception"), e. Lastly, they' 

 invariably pass through certain periodic changes in a definite and 

 discoverable order, — these changes constituting life. 



All the objects, then, which fulfil these conditions, are said to be 

 alive ; and they all belong either to the animal or to the vegetable 

 kingdom. The study of living objects of all kinds, irrespective of 

 which kingdom they behjiig to, is conveniently called by the general 

 name of Biology (Gr. hios, life ; and logos., discourse). As all living 

 objects, liowever, nuiy be referred to one or other of these two king- 

 doms, so Biology may be iliviiled into the two sciences of Botany, 

 which treats of plants, and Zoology (Gr. zoiin, animal ; logos, dis- 

 course), which treats of animals. The term Natural History, again, 

 is generally understood nowadays as being equivalent to Zoology 

 alone, though originally it was applied to the study of all natural 

 objects indiscriminately. 



2. Differences between Animals and Plants. 



It now becomes necessary to inquire into the differences which 

 subsist between animals and plants, and which enable us to separate 



