In relation with the external ear of man are found rudimentary muscles that are no longer useful and ordinarily are not under the control of the will. These muscles are the exact counterparts of well developed functional muscles found in great numbers of the lower animals. They are present in man as vestigial structures, because he is descended from animals in whom these muscles were well developed and functional.
The anatomy of man reveals so many additional vestigial structures that we may look upon him as a museum of obsolete anatomies; he is an old curiosity shop containing many discarded tools, many outgrown and aborted organs. The lower animals as well as man contain so many useless (vestigial) structures among their useful organs, and they are so significant of a former state of things in which they were useful, that anatomists are willing to stake the theory of evolution upon their presence alone. Evolution explains a multitude of other facts about man that are inexplicable on any other theory.
Fig. 19. Brain of Fish (Bluefish). A, dorsal view; B, side view; of, olfactory lobe; cr, cerebrum; ol, optic lobes; cb, cerebellum; m, medulla; th, thalami.
In addition to pointing out the possible track along which man has evolved from a primitive protozoan it would be interesting as well as exceedingly instructive to trace the development of each structure and organ in his body. But the subject is a vast one, and cannot be presented here even in briefest outline. Yet it will be very valuable to trace the unfolding of one organ, and that the highest, as a sample of what occurs with every part of the body. I refer to the development of the brain.
THE DEVELOPMENT OF THE BRAIN IN PHYLOGENY AND ONTOGENY.
Fig. 19 represents the brain of an average bony fish. It consists of six swellings in a line, one before the other. Beginning from the end towards the spinal cord, they are designated as follows, viz.: a single median lobe, the medulla (Metencephalon), m; then in front of this is another single median lobe, the cerebellum (Epencephalon), cb; then the optic lobes (Mesencephalon), right and left, ol; then the thalami (Thalamencephalon), which are small and hidden from view by the encroachment of the two adjacent segments; then the cerebrum (Prosencephalon), cr; then, finally, the olfactory lobes (Rhinencephalon), of. In this fish the largest of the segments are the optic lobes, ol.
Fig. 20.—Brain of Reptile (Turtle). A, dorsal view; B, side view; of, olfactory lobe; cr, cerebrum; th, thalami; ol, optic lobes; cb, cerebellum; m, medulla.
The reptile’s brain ([Fig. 20]) shows similar parts with the same serial arrangement. The reptile is a higher creature, a more intelligent animal, than the fish; and in consonance with this fact the cerebrum, cr, is the larger and more dominant part of the brain instead of the optic lobes, ol.