o would have been as exactly alike as
it is possible for two women to be, both mentally and physically;
even now they are constantly mistaken for one another."

In only a very few cases is some allusion made to the dissimilarity
being partly due to the combined action of many small influences,
and in none of the thirty-five cases is it largely, much less wholly,
ascribed to that cause. In not a single instance have I met with a
word about the growing dissimilarity being due to the action of the
firm free-will of one or both of the twins, which had triumphed over
natural tendencies; and yet a large proportion of my correspondents
happen to be clergymen, whose bent of mind is opposed, as I feel
assured from the tone of their letters, to a necessitarian view of
life.

It has been remarked that a growing diversity between twins may be
ascribed to the tardy development of naturally diverse qualities;
but we have a right, upon the evidence I have received, to go
farther than this. We have seen that a few twins retain their close
resemblance through life; in other words, instances do exist of an
apparently thorough similarity of nature, in which such difference
of external circumstances as may be consistent with the ordinary
conditions of the same social rank and country do not create
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 st