Even if we cannot accurately portray the anatomical details of a single ancestral stage, can we perhaps discover what function governed its life and was the aim of its existence? Did it live to eat, or to move, or to think? If we cannot tell exactly how it looked, can we tell what it lived for and what it contributed to the evolution of man? Now, the sequence of dominant functions or aims in life can be traced with far more ease and safety, not to say certainty, than one of anatomical details. The latter characterize small groups, genera, families, or classes; while the dominant function characterizes all animals of a given grade, even those which through degeneration have reverted to this grade. Even if I cannot trace the exact path which leads to the mountain-top, I may almost with certainty affirm that it leads from meadow and pasture through forest to bare rock, and thence over snow and ice to the summit; for each of these forms a zone encircling the mountain. Very similarly I find that, whatever genealogical tree I adopt, one sequence in the dominance of functions characterizes them all; digestion is dominant before locomotion and locomotion before 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 present. Hence--pardon the reiteration--if we can once trace this sequence of dominant functions, whose evolution has filled past ages, we can safely foretell something at least of man's future development.