IO Life and Immortality. 



their composition, and these elements are combined into 

 complex organic compounds, which always contain a large 

 percentage of water, are very unstable, and prone to spon- 

 taneous decomposition. They are composed of hetero- 

 geneous, but related, parts, termed organs, the objects 

 possessing them being called organized bodies. Some of 

 the lowest forms of animals have bodies whose substance is 

 so uniform that they exhibit no definite organs, but this 

 exception does not affect the general value of this distinc- 

 tion. They are always more or less definite in shape, 

 presenting concave and convex surfaces, and being limited 

 by curved lines. When they increase in size, or grow, as 

 we properly term it, it is not by the addition of particles 

 from the outside, but by the reception of foreign matter 

 into their interior and its consequent assimilation. Certain 

 periodic changes, which follow a definite and discoverable 

 order, are invariably passed through by organized bodies. 

 These changes constitute what is known as life. All the 

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

 alive, and they all appertain either to the vegetable or the 

 animal kingdom. The study of living objects, no matter 

 to which kingdom they belong, is therefore conveniently 

 called by the general name of Biology, which means a 

 discourse on life. And as all living objects may be referred 

 to one or other of these kingdoms, so Biology may be 

 divided into Botany, which treats of plants, and Zoology, 

 which treats of animals. 



Now that we have divided all organized bodies into plants 

 and animals, it becomes necessary to inquire into the differ- 

 ences which subsist between them, and which will enable 

 us to separate the kindred sciences of Botany and Zoology. 

 Nothing was thought so easy by older observers than the 

 determination of the animal or vegetable nature of any given 

 organism, but,. in point of fact, no hard-and-fast line can be 

 drawn, in the existing state of our knowledge, between the 

 animal and vegetable kingdoms, and it is sometimes difficult, 



