ents of
each stage have been clearly perceived. In other words, the first
prerequisite would seem to be a genealogical[A] tree of the animal
kingdom. The means of tracing this genealogical tree are given in
the first chapter, and the results in the second, third, and fourth
chapters of this book.

[Footnote A: See Phylogenetic Chart, p. 310.]

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