Now, for some of the ancestral stages of man’s development a very high degree of probability can be claimed.  One of man’s earliest ancestors was almost certainly a unicellular animal.  A little later he very probably passed through a gastraea stage.  He traversed fish, amphibian, and reptilian grades.  The oviparous monotreme and the marsupial almost certainly represent lower mammalian ancestral stages.  But what kind of fish, what species of amphibian, what form of reptiles most closely resembles the old ancestor?  How did each of these ancestors look?  I do not know.  It looks as if our ancestral tree were entirely uncertain and we were left without any foundation for history or argument.

But the history of the development of anatomical details, however important and desirable, is not the only history which can be written, nor is it essential.  It would be interesting to know the size of brain, girth of chest, average stature, and the features of the ancient Greeks and Romans.  But this is not the most important part of their history, nor is it essential.  The great question is, What did they contribute to human progress?

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.