mphioxus, as has been said, the
cranio-spinal axis alone exists; the Cyclostome fishes are but a step
higher. In fishes the true cerebrum appears at first in an insignificant
manner, a condition repeated in the early embryonic state both of birds
and mammals. An improvement is made in reptiles, whose cerebral
hemispheres are larger than their optic lobes. As we advance to birds, a
further increase occurs; the hemispheres are now of nearly sufficient
dimensions to cover over those ganglia. In the lower mammals there is
another step, yet not a very great one. But from the anterior lobes,
which thus far have constituted the entire brain, there are next to be
developed the middle lobes. In the Rodents the progress is still
continued, and in the Ruminants and Pachyderms the convolutions have
become well marked. [Sidenote: It attains its maximum in man.] In the
higher carnivora and quadrumana the posterior or tertiary lobes appear.
The passage from the anthropoid apes to man brings us to the utmost
development thus far attained by the nervous system. The cerebrum has
reached its maximum organization by a continued and unbroken process of
development.

[Sidenote: The same progressive development occurs in each individual
man.] This orderly development of the nervous system in the animal
series is recognized again in the gradual development of the individual
man. The primitive trace, as it faintly appears in the germinal
membrane, marks out the place presently to be occupied by the
cranio-spinal axis, and, that point of development gained, man answers
to the amphioxus. Not until the twelfth week of embryonic life does he
reach the state permanently presented by birds; at this time the
anterior lobes are only perceptible. In four or six weeks more the
middle lobes are evolved posteriorly on the anterior, and, finally, in a
similar manner, the tertiary or posterior ones are formed. And thus it
appears that, compared with the nervous system of other animals, that of
man proceeds through the same predetermined succession of forms. Theirs
suffers an arrest, in some instances at a lower, in some at a higher
point, but his passes onward to completion.

[Sidenote: It occurs again in the entire life of the globe.] But that is
not all. The biography of the earth, the life of the entire globe,
corresponds to this progress of the individual, to this orderly relation
of the animal series. Commencing with the oldest rocks that furnish
anim