2 INTRODUCTION. 
sciences of Geology, Mineralogy, Chemistry, and Physics. It 
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. 3. 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 “ organised.” Some of 
the lowest forms of animals have bodies composed of so uni- 
form a substance that they cannot be said to be organised, as 
they exhibit no definite organs. This exception, however, does 
not affect the general value of this distinction. ¢. They are al- 
ways more or less definite in shape, presenting concave and 
convex surfacés, and being bounded by curved lines. ad. 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 assimilation there (technically 
called intussusception”). ¢. Lastly, they invariably pass through 
certain periodic changes in a definite and discoverable order ; 
these changes constituting 7. 
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 belong to, is conveniently 
called by the general name of Biology (Gr. bios, lite; and logos, 
discourse). As all living objects, however, may be referred to 
one or other of these two kingdoms, so Biology may be divided 
* As will be mentioned immediately, it has been proposed to form an intermediate 
kingdom between the animal and vegetable kingdoms for the reception of organisms 
which cannot certainly be stated to be either plants or animals. There does not ap- 
pear, however, to be any necessity for his in the meanwhile, 
