to resemble one another, time and again, somewhat more closely than a brother and a sister. Furthermore, the two elder children and the two younger will tend to resemble one another more, as a rule, than the eldest resembles the youngest, and for a very sufficient reason, because all the habits and constitution of the two parents are liable to change from time to time, and especially after a long interval of years. Hence it will follow by parity of reasoning that two brothers or two sisters, born twins, will tend to resemble one another on the average far more intimately than do any two other members even of the same family. The rationale of this is clear. They are both the children of the one father and the one mother; they are both of the same sex; and they are both born at the same time, and therefore under exactly the same conditions of age, health, habit, and constitution on the part of both parents.
Here, then, we have a crucial instance by which we may test the physical and psychical correctness of this our general a priori principle. If character results in the way I say it does—if it is a product of the interaction of two independent sets of factors, derived equally on the whole from father and mother—then it will follow that, mentally and physically, twins will far more closely resemble one another than ordinary brothers and sisters do. Now, does the case of twins bear out in actual fact this debated deductive conclusion? Common experience tells us that it does, and Mr. Galton has supplemented that fallible and hasty guide by the most rigorous inductive collection of instances. The result of his investigation is simply this, that many twins do actually behave under similar circumstances in almost identical manners, that their characters often come as close to one another as it is possible for the characters of two human beings to come, and that even where the conditions of later life have been extremely different, the original likeness of type often persists to the very end, in spite of superficial variations in style or habit of living. Some of his stories, carefully verified, are very funny. I will supplement them by two of my own. In one case a couple of twins, men, had a quarrel over a perfectly unimportant matter. They came to very high words, and parted from one another in bad blood. On returning to their rooms—they lived apart—each of them suffered from a fit of remorse, and sat down to write a letter of contrition to the other, to be delivered by the morning post. After writing it one brother read his letter over, and, recalling the cause of quarrel, added at once a long postscript, justifying himself, and reopening the whole question at issue. The other brother posted his note at once, but thinking the matter over quietly, afterward regretted his action again, and supplemented it by a second palinodia, almost unsaying what he had said in the first one. I saw all three letters myself the next morning, and was simply amazed at their absolute sameness of feeling and expression.
The other story relates to a fact which happened, not to twins, but