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Inquiries into Human Faculty
dissimilarity. Positive evidence, such as this, cannot be outweighed by any
amount of negative evidence. Therefore, in those cases where there is a
growing diversity, and where no external cause can be assigned either by
the twins themselves or by their family for it, we may feel sure that it
must be chiefly or altogether due to a want of thorough similarity in their
nature. 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
