The characters which theoretically define a Mammal appear to be the enormous overgrowth of the cerebral hemispheres of the brain by which the cerebrum comes into contact with the cerebellum, as among Birds. This character distinguishes both groups of animals from all Reptiles, recent and fossil. But in examining the mould of the interior of the brain case it is rare to have the bones fitting so closely to the brain as to prove that the lateral expansion below the cerebrum and cerebellum is formed by the optic lobes of the brain. Otherwise the brain of a Pterodactyle might be as like to the brain of Ornithorhynchus as it is like that of a Bird (Fig. 19). But it is precisely in this condition of arrangement of the parts of the brain that the specimens appear to be most clear. The lateral mass of brain in specimens of Ornithosaurs from the Lower Secondary rocks appears to be transversely divided into back and front parts, which may be thought to correspond to the structures in a Mammal brain named _corpora quadrigemina_, but to be placed as the optic lobes are placed in Birds, and to have relatively greater dimensions than in Mammals. No evidence has been observed of this transverse division of the optic lobes of the brain in Pterodactyles from the Chalk and Cretaceous rocks, and so far as the evidence goes this part of the brain was shaped as in birds, but rather smaller. The brain is the only soft organ in which a Mammalian character could be evidenced. The uniformity in character of the brain throughout the group in Mammals is remarkable, in reference to the circumstance that the reproduction varies in type; the lowest, or Monotreme division, being oviparous. If there is no necessary connexion between the Mammalian brain and the prevalent condition under which the young are produced alive, it may be affirmed also that there is no necessary connexion between the form of the brain and the form of the bones, since the brain cavity in Theriodont reptiles shows no resemblance to that of a Mammal, while the bones are in so many respects only paralleled among Monotremata and Mammalia. The variety of forms which the existing Mammalian orders of animals assume, shows the astonishing range of structure of the skeleton which may coexist with the Mammalian brain. And therefore we are led to the conclusion that any other fundamental modification of brain--such as distinguishes the class of Birds--might also be associated with forms and structures of the skeleton which would vary in similar ways. In other words, if for convenience we define a Mammal by its form of brain, structure of the heart and lungs, and provision for nutrition of the young, without regard to the covering of the skin, which varies between the scales of a pangolin and the practically naked skin of the whale--a bird might be also defined by its peculiar conditions of brain and lungs, without reference to the feathered condition of the skin, though the feathered condition extends