78 APPLICATION OF NATURAL SYSTEM. 



obliged to examine its whole structure in order to discover its 

 place in the scale, we should subject ourselves to a vast amount 

 of labour, which must be a great impediment to the progress of 

 Zoology. This, however, is by no means the case ; for there are 

 certain external characters, simple and easily recognised, which 

 distinguish almost every class of animals from the rest; and which, 

 consequently, enable us to refer an unknown species to the pri- 

 mary group to which it belongs, with very little difficulty. Thus, 

 the Mammals are four-legged animals, whose bodies are more or 

 less closely covered with hair. There is no other class, in which 

 these two characters are united. All Reptiles, except Serpents, 

 are four-legged ; but their skins, instead of being beset with hair, 

 are either covered with scales, or are altogether naked. And, 

 again, some Insects and Spiders are hairy ; but they have never 

 so few as four legs. Hence, if we meet with a four-legged animal, 

 with hairs even very thinly scattered over its body, we may be 

 satisfied that it is a Mammal. Again, as there are no animals what- 

 ever, save Birds, that have their bodies covered with feathers, any 

 creature on whose surface such a covering exists, must be unques- 

 tionably placed in that class. Further, in the Articulated Series, 

 the higher classes may be recognised by the number of legs alone, 

 Insects having uniformly six, Spiders eight, and Crustacea not 

 less than ten. Hence, if we found an animal having six legs, we 

 should have no hesitation in stating it to belong to the class of 

 Insects, even though it may be destitute of wings, which we 

 commonly regard as characteristic of that group ; whilst any 

 animal with eight legs would be placed among the Spider tribe, 

 and any animal with ten would be associated with the Crabs and 

 Lobsters. If the number were greater than ten, it would be 

 necessary to examine the organs of respiration ; for, if these were 

 aquatic, the animal would belong to one of the lower groups of 

 the class Crustacea ; whilst if they were adapted to breathe air, 

 it would be one of the Myriapoda, or Centipede tribe. 



56. The same principle may be followed out in the subdivi- 

 sions of the classes ; so that, by a reference to the external cha- 

 racters alone of the animal examined, the Zoologist may be able 

 to predict all the essential points of its internal structure. Now 



