er do. Body
and soul are, for the working purposes of science, to be conceived
as in perfect accord, as co-helpers in the work of life, and as such
subject to a common development. Heredity, then, must be assumed to
apply to both equally. In proportion as there is plastic mind there
will be plastic body.

Unfortunately, the most plastic part of body is likewise the hardest
to observe, at any rate whilst it is alive, namely, the brain. No
certain criterion of heredity, then, is likely to be available from
this quarter. You will see it stated, for instance, that the size of
the brain cavity will serve to mark off one race from another. This
is extremely doubtful, to put it mildly. No doubt the average European
shows some advantage in this respect as compared, say, with the Bushman.
But then you have to write off so much for their respective types of
body, a bigger body going in general with a bigger head, that in the
end you find yourself comparing mere abstractions. Again, the European
may be the first to cry off on the ground that comparisons are odious;
for some specimens of Neanderthal man in sheer size of the brain cavity
are said to give points to any of our modern poets and politicians.
Clearly, then, something is wrong with this test. Nor, if the brain
itself be examined after death, and the form and number of its
convolutions compared, is this criterion of hereditary brain-power
any more satisfactory. It might be possible in this way to detect the
difference between an idiot and a person of normal intelligence, but
not the difference between a fool and a genius.

We cross the uncertain line that divides the bodily from the mental
when we subject the same problem of hereditary mental endowment to
the methods of what is known as experimental psychology. Thus acuteness
of sight, hearing, taste, smell and feeling are measured by various
ingenious devices. Seeing what stories travellers bring back with them
about the hawk-like vision of hunting races, one might suppose that
such comparisons would be all in their favour. The Cambridge Expedition
to Torres Straits, however, of which Dr. Haddon was the leader,
included several well-trained psychologists, who devoted special
attention to this subject; and their results show that the sensory
powers of these rude folk were on the average much the same as those
of Europeans. It is the hunter's experience only that enables him to
sight the game at an immense distance. There