so to regard himself were challenged, if he were called upon to justify
himself for having taken his foot as a unit of measurement, or his
fingers as the basis of his system of numbers, he might reply that
anything will serve as a standard for weights and measures, provided
that it never varies, but is always the same whenever referred to. But
the reply, valid though it is, does not do full justice to man: it
leaves room for the suspicion that a standard is something chosen by man
in a purely arbitrary manner and without reference to the facts of
nature. If that were really the case, then man's conception of himself
as superior to the other animals on earth might be but a prejudice of an
arbitrary kind. When, however, we consider, from the point of view of
evolution, the place of man among the other animals that occupy or have
occupied the earth, it is indubitable that the human organism is in
point of time the latest evolved and the human brain is in point of
complexity and efficiency the most highly developed. Further, the
evidence of embryology goes to show that the organism which has
eventually become human became so only by passing through successive
stages, each of which has its analogue in some of the existing forms of
animal life. Those forms of animal life exist side by side; and if we
conceive them to be represented diagrammatically by vertical lines,
differing in height according to their degree of evolution, the line
representing the human organism will be the tallest, and may be
considered to have become the tallest by successive increments or stages
corresponding to the height of the various other parallel vertical
lines.

When the conception of evolution, which had been employed to explain the
origin of species and the descent of man, and which had been gained by a
consideration of material organisms, came to be applied to the world of
man's thoughts, to the non-material and spiritual domain, and to be used