are a great many more
complicated tests of the same type designed to estimate the force of
memory, attention, association, reasoning and other faculties that
most people would regard as purely mental; whilst another set of such
tests deals with reaction to stimulus, co-ordination between hand and
eye, fatigue, tremor, and, most ingenious perhaps of all, emotional
excitement as shown through the respiration--phenomena which are, as
it were, mental and bodily at once and together. Unfortunately,
psychology cannot distinguish in such cases between the effects of
heredity and those of individual experience, whether it take the form
of high culture or of a dissipated life. Indeed, the purely temporary
condition of body and mind is apt to influence the results. A man has
been up late, let us say, or has been for a long walk, or has missed
a meal; obviously his reaction-times, his record for memory, and so
on, will show a difference for the worse. Or, again, the subject may
confront the experiment in very various moods. At one moment he may
be full of vanity, anxious to show what superior qualities he
possesses; whilst at another time he will be bored. Not to labour the
point further, these methods, whatever they may become in the future,
are at present unable to afford any criterion whatever of the mental
ability that goes with race. They are fertile in statistics; but an
interpretation of these statistics that furthers our purpose is still
to seek.

But surely, it will be said, we can tell an instinct when we come across
it, so uniform as it is, and so independent of the rest of the system.
Not at all. For one thing, the idea that an instinct is apiece of
mechanism, as fixed as fate, is quite out of fashion. It is now known
to be highly plastic in many cases, to vary considerably in individuals,
and to involve conscious processes, thought, feeling and will, at any
rate of an elementary kind. Again, how are you going to isolate an
instinct? Those few automatic responses to stimulation that appear
shortly after birth, as, for instance, sucking, may perhaps be
recognized, since parental training and experience in general are out
of the question here. But what about the instinct or group of instincts
answering to sex? This is latent until a stage of life when experience
is already in full swing. Indeed, psychologists are still busy
discussing whether man has very few instincts or whether, on the
contrary, he appears to have fe