Having now before us in outline a representation of the nervous system of man, and having in this a guide to the understanding of the prominent features involved in the distribution of two orders of nerves over the body, and their concentration in a central organ, we are prepared for considering the comparative brain development presented to view as we ascend the scale of animal life. The main features of gradation may be shortly stated. In all cases, the brain is a soft pulpy body, composed as described, the exterior portion being cellular tissue, the interior fibrous, from the gathering of nerve lines. In the lowest orders of animals, the brain is of very small size. In the _insects_, such as the ant, bee, and wasp, it is only a slight band stretching from eye to eye. In the whole order of _fishes_ an advance in organization appears, though the brain is small relatively to the size of the body, a fact which seems readily explained by the fact that there is little articulation in the structure of the fish, the whole body moving in one mass, by simple management of the fins and tail. The brain as a rule is simply two small round lobes of smooth surface laid together; and what is most to be remarked is that the brain proper is quite inferior in size to lobes of vision.[CT] In front of the brain are slight strands connected with the organ of smell; and behind it are the two large lobes known as optic lobes, before which the brain appears comparatively insignificant. This is the ordinary arrangement, but in the case of the shark the brain extends to much larger proportions, greatly surpassing the optic lobes, and having in front of it unusually ample provision for the organ of smell.[CU] When we reach the _reptiles_ the normal order appears which continues thereafter up the whole range of animal life. The brain takes precedence of the lobes of special sense, and is the most important organ. This appears quite decisively in the brain of the frog. On account of the possession of four limbs, and its power of locomotion by forward leaps, provided for by the superior size and strength of the hind legs, there is much greater need for distribution of nerve lines, to place distinct muscles under control, and as a consequence the brain or central organ assumes a position of greater importance. [Illustration: BRAIN OF THE BIRD.] [Illustration: BRAIN OF THE CAT, WITH BULB OF SMELL IN FRONT, AND LITTLE BRAIN BEHIND.]