thought.

And it is hardly less than a physiological necessity that it should
be so. The plant can and does exist, living almost purely for
digestion and reproduction, and the same is true of the lowest and
most primitive animals. A muscular system cannot develop and do its
work until some sort of a digestive system has arisen to furnish
nutriment, any more than a steam-engine can run without fuel. And a
brain is of no use until muscle and sense-organs have appeared.

This sequence of dominant functions,[A] of physiological dynasties,
would seem therefore to be a fact. And our series of forms described
in the second, third, and fourth chapters is merely a concrete
illustration showing how this sequence may have been evolved. The
substitution of other terms in the anatomical series there
described--amoeba, volvox, etc.--would not affect this result. By
a change in the form of our history we have eliminated to a large
extent the sources of uncertainty and error. And the dominant
function of a group throws no little light on the details of its
anatomy.

[Footnote A: See condensed Chart of Development, etc., p. 309.]

If we can be satisfied that ever higher functions have risen to
dominance in the successive stages of animal and human development,
if we can further be convinced that the sequence is irreversible, we
shall be convinced that future man will be more and more completely
controlled by the very highest powers or aims to which this sequence
points. Otherwise we must disbelieve the continuity of history. But
the germs of the future are always concealed in the history of the