have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that we can employ this method with the same
confidence and right as the geologist. No geologist has ever had
ocular proof that the vast rocks that compose our Carboniferous or
Jurassic or Cretaceous strata were really deposited in water. Yet no
one doubts the fact. Further, no geologist has ever learned by direct
observation that these various sedimentary formations were deposited
in a certain order; yet all are agreed as to this order. This is
because the nature and origin of these rocks cannot be rationally
understood unless we assume that they were so deposited. These
hypotheses are universally received as safe and indispensable
"geological theories," because they alone give a rational explanation
of the strata.

Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of "biological
theories" for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning
of the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in
certain quarters, will sooner or later be generally received. It is
true that, as will soon appear, our task is not so simple as that of
the geologist. It is just as much more difficult and complex as man's
organisation is more elaborate than the structure of the rocks.