importance in evolution, as long as it is a real factor, or which
theory of heredity or variation is the more probable.

If man has been evolved from simple living substance protoplasm, by
a process of evolution, it will some day be possible to write a
history of that process. But have we yet sufficient knowledge to
justify such an attempt?

Before the history of any period can be written its events must have
been accurately chronicled. Biological history can be written only
when the successive stages of development and the attainments 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 gastræa 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.