NATURAL SYSTEM OF CLASSIFICATION. 45 



tory their professed object of pursuit, it is equally advantageous 

 to gain a previous knowledge of the general plan of the Animal 

 Creation; since the characters of any particular division of it 

 are thereby much more completely understood, relations and 

 analogies of the greatest interest are discerned, which would have 

 been otherwise overlooked, and the labour bestowed, whether 

 on the examination of the structure, or on the systematic 

 arrangement, of the animals particularly sought for, is rendered 

 much more successful, by the guidance received from even 

 a very moderate general acquaintance with Zoology. 



24. The principles upon which Classification should be founded 

 in Zoology, are now generally admitted. It is not, however, 

 always easy to apply them. All Zoologists aim at constructing 

 a Natural System ; that is, a system which shall most fully de- 

 velope the general plan upon which the Creator has formed and 

 arranged the almost numberless species of animals, which owe 

 their existence to him. Now, this system must be constructed 

 in Zoology, as in Botany, by the careful examination of the whole 

 conformation of each species (BOTANY, 486) ; and by not resting 

 satisfied with superficial resemblances as indicating affinity, or 

 with variations of a really trivial, though perhaps very striking, 

 kind, as proofs of dissimilarity. Thus, for example, by the un- 

 informed, the Whale and its allies are commonly associated with 

 the class of Fishes, to which they bear a very obvious resemblance 

 in their aquatic habitation, and in their mode of propulsion 

 through the water ; whilst they are supposed to be distinct from 

 the Mammalia, with which they really correspond in all the 

 characters on which the Naturalist lays most stress, because they 

 live in a different element, and have bodies formed like those of 

 Fishes. 



25. Now, here we are led to perceive the difference between 

 characters that are essential, and those that are merely adaptive. 

 The essential characters, by which different classes are separated, 

 have all reference to the mode in which some or other of the 

 most important vital functions are performed. Thus, true Fishes 

 breathe by moans of gills, in which the blood is sufficiently acted 

 on by the air that is contained in the water around them : on 



