3'2 PRINCIPLES OF CLASSIFICATION. 



the spinal marrow and brain, the centres of the nervous sys- 

 tem ; and that it has five senses, four extremities, red blood, 

 &c. &c. Whilst the knowledge that it is one of the class Mam- 

 malia further implies, that it is a warm-blooded animal, breathing 

 air, possessing a heart with four cavities, a complete double 

 circulation, producing its young alive, nourishing them after- 

 wards by suckling, and having the body more or less completely 

 covered with hair. By being able to refer it to the order Car- 

 nivora, we know that it is a beast of prey (in its natural state), 

 adapted by the formation of its teeth and digestive apparatus to 

 feed upon animal flesh, and by the structure of its extremities to 

 pursue and attack the animals which serve as its prey. As one 

 of the family Canidae, we know that it agrees with the Wolves, 

 Foxes, Jackals, and Hyaenas, as well as with the Cats, in being 

 digitigrade, that is, in walking upon the ends of the toes, instead 

 of upon the sole of the foot, as does the Bear : but it agrees with 

 the former animals, and differs from the Cats, in not being so 

 much adapted to pursue and attack living prey, as to feed upon 

 the flesh of animals already killed ; the teeth not being formed 

 so exclusively for cutting and tearing, as are those of the Feline 

 or Cat tribe ; and the claws not being either so long and sharp 

 as theirs, or capable of being pushed forth and withdrawn, as in 

 the Cats. The dog differs from the Fox and Hyaena, in certain 

 peculiarities in the form of his teeth ; but his relationship to the 

 Wolf is so close, that many naturalists have regarded them as 

 having sprung from the same stock. Hence while the Dog be- 

 longs to a genus distinct from the Hyaena and Fox, it is included 

 with the Wolf and Jackal in the same generic character ; and it 

 may be, that it does not even constitute a species distinct from 

 the Wolf. 



12. There is frequently great difficulty, in Zoology as well 

 as in Botany, in distinguishing species from varieties. Any two 

 races of animals are considered to be of distinct species, which 

 are marked by characters of difference that are constantly 

 exhibited ; so that neither shows any tendency to lose its own 

 peculiarity, or to acquire that of the other. Thus, notwith- 

 standing the variety of forms exhibited by the several races 



