THE HISTORY OF TWINS, ETC. 355 



grow unlike through the development of natural characteristics which 

 had lain dormant at first, or else they continue their lives, keeping 

 time like two watches, hardly to be thrown out of accord except by 

 some physical jar. Nature is far stronger than nurture within the 

 limited range that I have been careful to assign to the latter. 



The effect of illness, as shown by these replies, is great, and well 

 deserves further consideration. It appears that the constitution of 

 youth is not so elastic as we are apt to think, but that an attack, say 

 of scarlet fever, leaves a permanent mark, easily to be measured by 

 the present method of comparison. This recalls an impression made 

 strongly on my mind several years ago by the sight of a few curves 

 drawn by a mathematical friend. He took monthly measurements of 

 the circumference of his children's heads during the first few years 

 of their lives, and he laid down the successive measurements on the 

 successive lines of a piece of ruled paper, by taking the edge of the 

 paper as a base. He then joined the free ends of the lines, and so 

 obtained a curve of growth. These curves had, on the whole, that 

 regularity of sweej) that might have been expected, but each of them 

 showed occasional halts, like the landing-places on a long flight of 

 stairs. The development bad been arrested- by something, and was 

 not made up for by after-growth. Now, on the same piece of paper 

 my friend had also registered the various infantine illnesses of the 

 children,- and corresponding to each illness was one of these halts. 

 There remained no doubt in my mind that, if tliese illnesses had been 

 warded off, the development of the children would have been in- 

 creased by almost the precise amount lost in these halts. In other 

 words, the disease had drawn largely upon the capital, and not only 

 on the income, of their constitutions. I hope these remarks may in- 

 duce some men of science to repeat similar experiments on their chil- 

 dren of the future. They may compress two years of a child's his- 

 tory on one side of a ruled half-sheet of foolscap paper if they cause 

 each successive line to stand for a successive month, beginning from 

 the birth of the child ; and if they mark off the measurements by lay- 

 ing, not the 0-inch division of the tape against the edge of the pages, 

 but, say, the 10-inch division in order to economize space. 



The steady and pitiless march of the hidden weaknesses in our 

 constitutions, through illness to death, is painfully revealed by these 

 histories of twins. We are too apt to look upon illness and death as 

 capricious events, and there are some who ascribe them to the direct 

 effect of supernatural interference, whereas the fact of the maladies 

 of two twins being continually alike shows that illness and death are 

 necessary incidents in a regular sequence of constitutional changes, 

 beginning at birth, upon Avhich external circumstances have, on the 

 whole, very small effect. In cases where the maladies of the twins 

 are continually alike, the clock of life moves regularly on, governed 

 by internal mechanism. When the hand approaches the hour-mark, 



