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, 
