these. This process is repeated again and again until groups of animals are reached in which the differences of structures separating them from others become so minute that they are called varietal or specific while the differences of environment to which these subdivisions are adapted grow fainter and fainter, until, when the point is reached where the classifier is compelled to throw together a number of animals and call them a species, because he can no longer find structural differences to form divisions upon, there the student of animal habits will find the same animals related to practically identical facts of environment.
An example of such changes in environment and of such following modifications of structures may be taken from the mammals. Primarily arboreal in habits, and climbing, with hands developed on one or both pairs of limbs, their skulls and jaws and teeth were fitted for masticating their food. All of their supposed ancestry had simply swallowed their food whole.
This mammalian character of the teeth, when attained to, made of importance, for the first time, such differences of foods as necessitated different kinds of teeth for masticating them. Examined in this respect, the food ordinarily made use of by mammals may be roughly divided into five classes—fruits, insects, flesh, grass, and hard foods—and these have led to the formation of the five great orders: primates, insectivores, carnivores, ungulates, and rodents. The importance of these food characteristics in modifying mammals may be seen when it is stated that these five orders, with the bats, which are flying insectivores, contain five sixths of the mammalian species and probably more than nineteen twentieths of all the individuals of the class. Again, the becoming fitted for hard foods, by the rodents, brought them under the influence of a new set of surroundings, namely, the various locations in which such foods existed, and the families of squirrels, mice, hares, beavers, etc., are the result. The same process again took place in the formation of the genera and species of these families.
We must look upon the class, order, family, genus, and species characters of each individual animal as structures which fit it or have fitted its ancestors for as many distinct facts of environment. The later gained and more trivial structures may overlie and obscure the more ancient and fundamental ones, like the later writings on an old palimpsest, but all are to be made out by the skillful anatomist. As has been said, the line of modification is not always a direct one, but it is often so deflected that structures which were primarily adapted to one fact of environment may be modified secondarily to fit them for others which are opposed and antagonistic to the first. The
