These deepest-seated parts of the limbs are slow to adapt themselves to changing circumstances of existence, and retain their characters with moderate variation of the bones in each of the orders or classes of animals. It therefore is safer to regard Mammalian characters, as well as the resemblances which Pterodactyles show to other kinds of animals, as due to inheritance from a time when there was a common stock from which none of these animals which have been considered had been distinctly elaborated. A few characters of Ornithosaurs are regarded as having been acquired, because they are not found in any other animals, or have been developed only in a portion of the group. The most obvious of these is the elongated wing finger; but in some genera, like Dimorphodon, there is also a less elongation of the fifth digit of the foot, and perhaps in all genera there is a backward development of the first digit of the hand, which is without a claw, and therefore unlike the clawed digit of a Bat. An acquired character of another kind, which is limited to the Cretaceous genera, is seen in the shoulder-blade being directed transversely outward, so that its truncated end articulates by a true joint with the early vertebræ of the back, and defended the cavity inclosed by the ribs by a strong bony external arch. And finally, as the animals later in time acquire short tails, and relatively longer limbs, the bones of the back of the hand, termed metacarpals, acquire greater and distinctive length, which is not seen in the long-tailed types like Rhamphorhynchus. These and such-like acquired characters distinguish the class of animals from all groups with which it may be compared, and mark the possible limits of variation of the skeleton within the boundary of the order. But no further variation of these parts of the skeleton could make a transition to another order of animals, or explain how the Pterodactyles came into existence, because the characters which separate orders and classes of animals from each other differ in kind from those which separate smaller groups, named genera and species, of which the order is made up. The accumulation of the characters of genera will not sum up into the characters of an order or class. In making the division of Vertebrate animals into classes the skeleton is often almost ignored. Its value is entirely empirical and based upon the observed association of the various forms of bones with the more important characters of the brain and other vital organs. What is understood as a Mammalian or Avian character in the skeleton is the form of bone which is found in association with the soft vital organs which constitute an animal a Mammal or a Bird.