126 AN INTRODUCTION TO ZOOLOGY. 



So much for the manner of classification; a -word must 

 be said about its meaning. All classifications have been en- 

 deavours to present the members of the animal kingdom in 

 groups according to their more or less lilceness with one 

 another, and to arrange these groups in a gradual series link- 

 ing the highest and the lowest forms.' The older writers, 

 however, in arranging their series, preferred to present it in 

 inverse order, i.e. beginning with the highest types ; and they 

 did not place among inquiries falling within the scope of 

 human knowledge, those questions which would so naturally 

 suggest themselves to us, — as to liow and wUy these like- 

 nesses a.nd differences exist among animals, and why the 

 groups of the animal kingdom should be capable of being 

 arranged in a graduated series. Makers of classification in 

 the present day understand, on the contrary, that classifica- 

 tion implies an answer to these questions. Likeness of 

 structure is understood to imply community of descent. The 

 graduated series of animal forms is understood to present the 

 traces, preserved among surviving forms, of the evolution of 

 animal life through a series of types, of which many have been 

 lost, while others still survive. From this point of view, one 

 single character may be of more importance in determining 

 the position of an animal in classification than all its other 

 organs. 



It will be understood, from the above considerations, that a 

 good deal must be learnt about zoology before the simplest 



don': Parlcer & Sons, 1858), from wliicb the above quotations are takeu. 

 Such portions of Whewell's " History of the Inductive Sciences " as refer 

 to the same subject may also be consulted, bearing in mind, however, that 

 the scientific classifications referred to are those of a past date. 



' Although the groups of the animal kingdom may thus be so arranged 

 in a linear series, it must be borne in mind that such a series, so far as it 

 is linear, is_ artificial. Just as, in the tree of our diagram, the highest 

 twia belonging to a lower bough may rise above the lowest twig belong- 

 ing to a higher bough, so the highest members of a low group may roach 

 a point of complexity of organization superior to that obtained by the 

 lower members of a higher group. Therefore a linear series, snch as we 

 have in the successive pages of a text-book of zoology, will not represent 

 thd complexity of these relationships, which can only be illustrated by a 

 map or diagram. 



