/O 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, 

 and a few Lizards, are four-legged ; but their skins, instead of 

 being beset with hair, are covered either with scales or with 

 bony plates. 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 whatever, save Birds, that have their 

 bodies covered with feathers, any creaturr on whose surface such 

 a covering exists, must be unquestionably placed in that class. 

 Further, in the Articulated Series, the higher classes may be re- 

 cognised 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 ;* 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. 



06. 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 



* It is to be observed, however, thut the young oi'many mites have only six legs 



