under the earth,—­by the number of young they bring forth,—­by their different seasons of breeding,—­and by still minor differences, such as the permanent color of the hair throughout the year in some, while in others it turns white in winter.  The Rats and Mice differ in a similar way:  there being large and small Species,—­some gray, some brown, others rust-colored,—­some with soft, others with coarse hair; they differ also in the length of the tail, and in having it more or less 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 limited 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 Genera, 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 Genera would show us that among Birds, Reptiles, Fishes, and, in fact, throughout the Animal Kingdom, Species of well-defined natural Genera differ 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 combined under certain minor differences of size, proportion, and habits.  All the specific characters stand in direct reference to the generic structure, the family form, the ordinal complication of structure, the mode of execution of the Class, and the plan of structure of the Branch, all of which are embodied in the frame of each individual in each Species, even though all these individuals are constantly dying away and reproducing others; so that the specific characters have no more permanency in the individuals than those which characterize the Genus, the Family, the Order, the Class, and the Branch.  I believe, therefore, that naturalists have been entirely wrong in considering the more comprehensive groups to be theoretical and in a measure arbitrary, an attempt, that is, of certain men to classify the Animal Kingdom according to their individual views, while they have ascribed to Species, as contrasted with the other divisions, a more positive existence in Nature.  No further argument is needed to show that it is not only the Species that lives in the individual, but that every individual, though belonging to a distinct Species, is built upon a precise and definite plan which characterizes its Branch,—­that that plan is executed in each individual in a particular