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 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