Passing next to _birds_, we find a marked advance in the structure of the brain. The two hemispheres are considerably extended towards the rear, and the two optic lobes underneath the back part of the brain are separated from each other, being placed somewhat to the side. The cerebellum, or little brain, regulating equilibrium becomes more important in size and form, being laid up in transverse furrows. These important advances indicate a life of much more varied activity than in the lower orders. This animal walks, hops, perches on branches by the clutching of its claws, and flies from place to place. To provide for these varied forms of activity, there must be a more detailed arrangement of nerve system, which is clearly indicated in the complexity of the central organ. The next advance introduces to notice the _smaller quadrupeds_, known as the rodents, of which the rat, rabbit, and hare may be taken as the most familiar examples. Here we still have the smooth surface of the brain, without any subdivision and twining into folds such as afterwards appears, but it is somewhat elongated in shape. An additional element here comes into view, that is, extra provision for acuteness of smell, in accordance with the well-known characteristics of the class of animals. Set out in front of the brain are two distinct lobes, which are the olfactory lobes. Wherever these are so placed in front of the brain, it is a clear proof that the life of the animal is largely directed by smell, that is, in a relatively greater degree than by sight, though constantly using the organs of vision with rapidity and acuteness. The cerebellum is in all cases prominent to the rear, presenting the laminated appearance always distinctive of the organ. We now make a very marked transition in the development of brain, introducing to view the doubled or convoluted form occasioned by the folding of the material in a series of windings,--a form which is in complete contrast from the smooth surface characteristic of the brain in all lower orders. This series of windings or convolutions appears quite decidedly in the brain of the _cat_, in a manner very similar in the brain of the _dog_, and with still greater beauty and amplitude of fold in the brain of the _horse_. This folding process which is resorted to in the case of all the higher quadrupeds, seems a contrivance by which it is possible to pack a greater amount of material in such a way as to expose a greater degree of surface, within the narrow space at command inside the cranium. In all the three examples named, great prominence is given to the bulbs of smell, which are spread out quite conspicuously in front of the brain,--implying, as in lower examples, a life largely governed by sense of smell. [Illustration: BRAIN OF HORSE, WITH BULB OF SMELL IN FRONT, LITTLE BRAIN IN REAR.]