But not only is this order found in the evolution of the whole vertebrate department, but something of the same kind is found also in the evolution of _each class_. The earliest reptiles, the earliest birds, and the earliest mammals had smaller and less perfectly organized brains than their nearest congeners of the present day. This is shown in the accompanying figures (Figs. 57 and 58). To carry out one example more perfectly: In the history of the horse family, in connection with the changes of skeletal structure already described (page 108), we have also corresponding changes in the size and structure of the brain; _pari passu_ with the improvement of the mechanism we have also increased engine-power and increased muscular energy and therefore increased activity and grace. The brain of a modern horse, though not very large, is remarkable for the complexity of its convolutions. The great energy, activity, and nervous excitability of the horse are the result of this structure. =Cephalization.=--Thus, in going up the phylogenic, the taxonomic, or the ontogenic series, we find a gradual process of development headward, brainward, cerebrumward; or, more generally, we might say that in all organic evolution we find an increasing dominance of the higher over the lower, and of the highest over all. For example, in the lowest plane of either series we find first the different systems imperfectly or not at all differentiated. Then, as differentiation of these progress, we find an increased dominance of the highest system--the _nervous system_; then in the nervous system, the increasing dominance of its highest part--the _brain_; then in the brain the increasing dominance of its highest ganglion--the _cerebrum_; and, lastly, in the cerebrum the increasing dominance of its highest substance--the exterior gray matter--as shown by the increasing number and depth of the convolutions. This whole process may be called _cephalization_. Shall the process stop here? When evolution is transferred from the animal to the human plane, from the physiological to the psychical, from the involuntary and necessary to the voluntary and free, shall not the same law hold good? Yes! all social evolution, all culture, all education, whether of the race or the individual, must follow the same law. All _psychical advance is a cephalization_--i. e., an increasing dominance of the higher over the lower and of the highest over all; of the mind over the body, and in the mind of the higher faculties over the lower; and, finally, the subordination of the whole to the highest moral purpose. [Illustration: FIG. 59.--Homocercal tail-fin. A, form; B, structure.] [Illustration: FIG. 60.--Heterocercal or vertebrated tail-fin. A, form; B, structure.]