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.
Fig. 59.—Homocercal tail-fin. A, form; B, structure.
Fig. 60.—Heterocercal or vertebrated tail-fin. A, form; B, structure.
4. Fish-Tails.—Still another and last example: It has long been noticed that there are among fishes two styles of tail-fins. These are the even-lobed, or homocercal ([Fig. 59]), and the uneven-lobed, or heterocercal ([Fig. 60]). The one is characteristic of ordinary fishes (teleosts), the other of sharks and some other orders. In structure the difference is even more fundamental than in form. In the former style the backbone stops abruptly in a series of short, enlarged joints, and thence sends off rays to form the tail-fin ([Fig. 59], B); in the latter the backbone runs through the fin to its very point, growing slenderer by degrees, and giving off rays above and below from each joint, but the rays on the lower side are much longer ([Fig. 60], B). This style of fin is, therefore, vertebrated, the other non-vertebrated. [Figs. 59] and [60] show these two styles in form and structure. But there is still another style found only in the lowest and most generalized forms of fishes. In these the tail-fin is vertebrated and yet symmetrical. This style is shown in [Fig. 61], A and B.
Fig. 61.—Vertebrated but symmetrical fin. A, form; B, structure.
Now, in the development of a teleost fish ([Fig. 58]), as has been shown by Alexander Agassiz,[24] the tail-fin is first like [Fig. 61]; then becomes heterocercal, like [Fig. 60]; and, finally, becomes homocercal like [Fig. 59]. Why so? Not because there is any special advantage in this succession of forms; for the changes take place either in the egg or else in very early embryonic states. The answer is found in the fact that this is the order of change in the phylogenic series. The earliest fish-tails were either like [Fig. 61] or [Fig. 60]; never like [Fig. 59]. The earliest of all were almost certainly like [Fig. 61]; then they became like [Fig. 60]; and, finally, only much later in geological history (Jurassic or Cretaceous), they became like [Fig. 59]. This order of change is still retained in the embryonic development of the last introduced and most specialized order of existing fishes. The family history is repeated in the individual history.