THE CELL AND ITS STRUCTURE. 27 



acters belonging to the adult form. If we arrange the 

 various types of the animal kingdom in order, we may 

 form them into a series somewhat comparable to the 

 series of different stages shown by the young animal in 

 the Q^^. That is to say, we may form a series of which, 

 the first and simplest kind consists merely of one cell, 

 while the successive members of the series become by 

 degrees more and more complicated. Only the series 

 will not present one animal belonging to each stage of 

 complication, but many and varied types belonging to 

 each stage. We shall obtain a fairly correct idea of 

 the necessary classification, if we imagine it repre- 

 sented as a tree, with branches, twigs, and leaves, of 

 different ages, and situated at various levels; the 

 lower levels representing the oldest and simplest 

 types, the higher levels the newest and most com- 

 plicated types. We cannot frame a classification by 

 which we shall pass simply step by step from one 

 stage of animal type to the next higher stage ; but 

 we have to deal with collateral branches, and with 

 all sorts of twigs and offsets from them (p. 305). 



In order to understand classification, we must first 

 understand something about the different parts and 

 organs which go to make up the animal body. As 

 has been already said, an organ is a part of an animal 

 consisting primarily of a group of cells of similar 

 kind, which are modified so as to be especially suited 

 to perform some special portion of the work of main- 

 taining life. The study of the uses of organs is called 

 Fhysiology ; its object is, to find out what are 



