general mode of execution,--nothing of the plan expressed in that mode of execution. Yet, with the exception of the ordinal characters, which, since they imply relative superiority and inferiority, require, of course, a number of specimens for comparison, his one animal would tell him all this as well as the specific characters. All the more comprehensive groups, equally with Species, have a positive, permanent, specific principle, maintained generation after generation with all its essential characteristics. Individuals are the transient representatives of all these organic principles, which certainly have an independent, immaterial existence, since they outlive the individuals that embody them, and are no less real after the generation that has represented them for a time has passed away than they were before. From a comparison of a number of well-known Species belonging to a natural Genus, it is not difficult to ascertain what are essentially specific characters. There is hardly among Mammalia a more natural Genus than that which includes the Rabbits and Hares, or that to which the Rats and Mice are referred. Let us see how the different Species differ from one another. Though we give two names in the vernacular to the Genus Hare, both Hares and Rabbits agree in all the structural peculiarities which constitute a Genus; but the different Species are distinguished by their absolute size when full-grown,--by the nature and color of their fur,--by the size and form of the ear,--by the relative length of their legs and tail,--by the more or less slender build of their whole body,--by their habits, some living in open grounds, others among the bushes, others in swamps, others burrowing 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