Why, with a thousand red and a thousand white balls, shaken together with an equal energy by a machine (if you will), and poured out on our billiard-table, should there be a similar diversity? The fact is, you can not get absolute identity of conditions in any two cases. Imagine yourself mixing two fluids together with a spoon, as regularly as you choose; can you possibly make the currents in the two exactly alike twice running? And here in the case of humanity you have not to deal with simple red beans or with simple fluids, but with very complex gemmules or very complex physiological units.
If even in twins we can not expect perfect similarity, still less can we expect it in mere ordinary brothers and sisters. Here, innumerable minor physiological conditions of either parent may affect the result in infinite ways. Not, indeed, that there is any sufficient reason for supposing passing states of health and so forth directly to impress themselves upon the heredity of the offspring; but one can readily understand that, in a process which is essentially a mixture of elements, small varieties of external circumstances may vastly alter the nature of the result. Shake the bag of beans once, and you get one arrangement; shake it once more, and you get another and very different one. To this extent, and to this extent only, as it seems to me, chance in the true sense enters into the composition of an individuality. The possible elements which may go to make up the mental constitution of any person are (as I shall try to show) strictly limited to all those elements, actual or latent, which exist in the two persons of his parents; but the particular mixture of those elements which will come out in him—the number to be selected and the number to be rejected out of all the possible combinations—will depend upon that minute interaction of small physical causes, working unseen, which we properly designate by the convenient name of chance. In this sense, it is not a chance that William Jones, the son of two English parents, is born an Englishman in physique and mental peculiarities, rather than a Chinese or a negro; nor is it a chance that he is born essentially a compound of his ancestors on the Jones side and on the Brown side, but it is a chance that he is born a boy rather than a girl; and it is a chance that he is born himself rather than his brother John or his brother Thomas. If we knew all, we could point out exactly why this result and not any other result occurred just there and then; but, as we do not know all, we fairly say that the result is in so far a chance one. And, even if we knew all, we should still be justified in using the same language, for it marks a real difference in causation. William Jones is an Englishman and a Jones-Brown strictly in virtue of his being the son of Henry Jones and Mary Brown; but so are all his brothers and (mutatis mutandis) his sisters too. He is himself, and not one of his brothers or his sisters, in virtue of certain minute molecular arrangements, occurring between certain elements for the most part essentially identical with the elements which went to make up
