nge of people, until it passed into the common
knowledge of the world, that confusion of which we have spoken arose
between evolution and Darwin's particular theory. And as knowledge grew,
and the number of biologists increased in the striking fashion of this
last half-century, while the evidence for evolution continued to
increase with an unexpected rapidity, every detail of the purely
Darwinian theory became more and more subjected to rigid scrutiny. Most
educated people, unless their education has been largely in an
experimental science, find difficulty in understanding the relation in
the minds of naturalists between "authority" and "knowledge." We do not
_know_, for instance, that the structure of the Medusae consists
essentially of two foundation-membranes, because Huxley, one of the
greatest authorities in anatomy that the world has seen, told us that it
was so. We know it because, Huxley having told us that it was so, we are
able at any time with a microscope and dissecting needles to observe the
fact for ourselves. It is true, that unless we are making a special
study of the Medusae we do not repeat the observation in the case of so
many different forms of Medusae as Huxley studied; but it is part of our
training to observe for ourselves in a sufficient number of cases to
test the correspondence between statement and fact before we accept the
generalisation of any authority. And we learn, or at least have the
opportunity of learning, in the whole habit of our lives as naturalists,
to distinguish carefully between knowledge of which personal observation
is an essential part, and opinion or belief which may or may not be
based upon authority, but which in any case is devoid of the
corroboration of personal observation. When a piece of new anatomical or
physiological work is published in a technical journal, it is read by a
large number of anatomists and physiologists, and if the work is
apparently of an important kind, bearing on the general problems that
even specialists have to follow, they all at once set to work in their
laboratories to make corroborative dissections or experiments, and it is
part of every modern account of a biological discovery to tell exactly
the methods by which results were got, in order that this process of
corroboration may be set about easily. The question as to whether or no
natural selection were the sole or chief cause, or indeed a cause at
all, of evolution is not yet, and perhaps n