Other methods, however, may supplement the pathological one. The comparative anatomist shows us that the development of the central nervous system in the kingdom of animals goes parallel to the development of the mental functions, and that it is not only a question of progress along all lines. Any special function of the mind may have in certain animal groups an especially high development, and we see certain parts correspondingly developed. The dog has certainly a keener sense of smell than the man--the part of the brain which is in direct connection with the olfactory nerve is correspondingly much bulkier in the dog's brain than in the human organism. Here too, of course, research may be carried to the subtlest details and the microscope has to tell the full story. Not the differences in the big structure, but the microscopical differences in the brain cells of special parts are to be held responsible. But comparison may not be confined to the various species of animals; it may refer not less to the various stages of man. The genetic psychologist knows how the child's mind develops in a regular rhythm, one mental function after another, how the first days and first weeks and first months in the infant's life have their characteristic mental possibilities, and no mental function can be anticipated there. The new-born child can taste milk, but cannot hear music. The anatomist shows us that correspondingly only certain nervous tracts have the anatomical equipment by which they become ready for functioning. Most of the tracts at first lack the so-called medullar sheath, and from month to month new paths are provided with this physical equipment. Finally we have the experiment of the physiologist. His vivisectional experiments, for instance, demonstrate that the electrical stimulation of a definite spot on the surface of a dog's brain produces movements which we should ordinarily take as expressions of mental states, movements of the front legs or of the tail, movements of barking or whining. On the other hand, the dog becomes unable to fulfill the mental impulses if certain definite parts of his brain are destroyed. The physiologist may show from the monkey down to the pigeon, to the frog, to the ant, to the worm, how the behavior of animals is changed as soon as certain groups of nervous elements are extirpated. It is the mental emotional character of the pigeon which is changed when the physiologist cuts off parts of his brain. In short, stimulation and destruction demonstrate, by experiments which supplement each other, that mental functions correspond to brain functions.