138 SPECIES AND BREEDS. 



covered with hair, in the cut of the ears, and 

 their size, in the length of their limbs, which 

 are slender and long in some, short and thick in 

 others, in their various ways of living, in the 

 different substances on which they feed, and 

 also in their distribution over the surface of the 

 earth, whether circumscribed within certain lim- 

 ited areas or scattered over a wider range. 



What is now the nature of these differences by 

 which we distinguish Species ? They are totally 

 distinct from any of the categories on which Gen- 

 era, Families, Orders, Classes, or Branches are 

 founded, and may readily be reduced to a few 

 heads. They are differences in the proportion of 

 the parts and in the absolute size of the whole 

 animal, in the color and general ornamentation 

 of the surface of the body, and in the relations 

 of the individuals to one another and to the 

 world around. A farther analysis of other Gen- 

 era would show us that among Birds, Reptiles, 

 Fishes, and, in fact, throughout the Animal King- 

 dom, Species of well-defined natural Genera dif- 

 fer in the same way. We are therefore justified 

 in saying that the category of characters on 

 which Species are based implies no structural 

 differences, but presents the same structure com- 

 bined under certain minor differences of size, 

 proportion, and habits. All the specific characters 

 stand in direct reference to the generic structure, 



