Nay, further, in some cases it is distinctly affirmed that the growing dissimilarity can be accounted for in no other way.  We may, therefore, broadly conclude that the only circumstance, within the range of those by which persons of similar conditions of life are affected, that is capable of producing a marked effect on the character of adults, is illness or some accident which causes physical infirmity.  The twins who closely resembled each other in childhood and early youth, and were reared under not very dissimilar conditions, either 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 some 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 sweep that might have been expected, but each of them showed occasional halts, like the landing-places on a long flight of stairs.  The development had 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 infantile illnesses of the children, and corresponding to each illness was one of these halts.  There remained no doubt in my mind that, if these illnesses had been warded off, the development of the children would have been increased 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 induce some men of science to repeat similar experiments on their children of the future.  They may compress two years of a child’s history 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 economise space by laying, not the 0-inch division of the tape against the edge of the pages, but, say, the 10-inch division.