INTEODUCTION. XI 



Such changes are always definite and follow one 

 another in a determinate order. This is the most 

 general and characteristic feature of living beings, 

 since matter, of itself, if unacted on from without, 

 is incapable of undergoing any change. 



Other peculiarities, of minor importance, dis- 

 tinguish living from inorganic bodies. Of these, 

 the principal have reference to external form, 

 internal structure, cheriiiical constitution, and 

 mode of increase. 



1. External form. — The figure of living beings 

 is always more or less rounded, being bounded by 

 convex surfaces. That of inorganic bodies, on the 

 contrary, is either indefinite (amorphous), or, if 

 regular (crystalline), is, mth very few exceptions, 

 bounded by angles and right lines. 



2. Internal structure. — The structure of a 

 mineral body, provided it be pure and unmixed 

 with any other substance, is altogether homo- 

 geneous, consisting throughout of an aggi'egation 

 of similar particles. Such a body, therefore, can- 

 not properly be said to possess any structure 

 whatsoever, whereas the body of a living being 

 usually consists of several parts, distinct from one 

 another, which, again, in their turn, are seen to 

 be composed of more minute constituents or tissues, 

 these last being yet again resolvable into certain 

 ultimate elementary components. Hence their 

 structure is said to be heterogeneous, and the ex- 

 pression organised bodies has come to be synony- 



