is to kill it once and for all.  Life, as was said in the first chapter, is something unique, with the unique property of being able to evolve.  As life evolves, that is to say changes, by being handed on from certain forms to certain other forms, a partial rigidity marks the process together with a partial plasticity.  There is a stiffening, so to speak, that keeps the life-force up to a point true to its old direction; though, short of that limit, it is free to take a new line of its own.  Race, then, stands for the stiffening in the evolutionary process.  Just up to what point it goes in any given case we probably can never quite tell.  Yet, if we could think our way anywhere near to that point in regard to man, I doubt not that we should eventually succeed in forging a fresh instrument for controlling the destinies of our species, an instrument perhaps more powerful than education itself—­I mean, eugenics, the art of improving the human breed.

To see what race means when considered apart, let us first of all take your individual self, and ask how you would proceed to separate your inherited nature from the nature which you have acquired in the course of living your life.  It is not easy.  Suppose, however, that you had a twin brother born, if indeed that were possible, as like you as one pea is like another.  An accident in childhood, however, has caused him to lose a leg.  So he becomes a clerk, living a sedentary life in an office.  You, on the other hand, with your two lusty legs to help you, become a postman, always on the run.  Well, the two of you are now very different men in looks and habits.  He is pale and you are brown.  You play football and he sits at home reading.  Nevertheless, any friend who knows you both intimately will discover fifty little things that bespeak in you the same underlying nature and bent.  You are both, for instance, slightly colour-blind, and both inclined to fly into violent passions on occasion.  That is your common inheritance peeping out—­if, at least, your friend has really managed to make allowance for your common bringing-up, which might mainly account for the passionateness, though hardly for the colour-blindness.

But now comes the great difficulty.  Let us further suppose that you two twins marry wives who are also twins born as like as two peas; and each pair of you has a family.  Which of the two batches of children will tend on the whole to have the stronger legs?  Your legs are strong by use; your brother’s are weak by disuse.  But do use and disuse make any difference to the race?  That is the theoretical question which, above all others, complicates and hampers our present-day attempts to understand heredity.