DISTINCTION BETWEEN SPECIES AND VARIETIES. 33 



of Dog, we never see any which present so strong a resemblance 

 to a Fox, as to be at all in danger of being mistaken for that 

 animal ; and they may always be distinguished by this obvious 

 character, that the pupil of the eye of the Dog is always round, 

 whilst that of the Fox is oval when contracted (ANIM. PHYSIOL. 

 533). When such constant distinctions are found to exist, the 

 Zoologist has a right to assume that they always have existed ; 

 and consequently that the original parents, or the stocks whence 

 the races originated, were also distinct. 



13. But whilst some characters are constant in each race, 

 others may undergo great variation ; so that, within the limits 

 of one species, we may have a large number of varieties or breeds, 

 marked by differences much greater than those which, in other 

 cases, are held sufficient to distinguish species. This is especially 



FIG. 1. THE HIGHLAND GREYHOUND. 



the case with our various domesticated animals ; and indeed it is 

 in great part from the power which their constitutions possess, 



