her, every expression in the one may be
matched by a corresponding expression in the other, but it does not
follow that the same expression should be the prevalent one in both
cases. Now it is by their prevalent expressions that we should
distinguish between the twins; consequently when one twin has
temporarily the expression which is the prevalent one in his brother,
he is apt to be mistaken for him. There are also cases where the
development of the two twins is not strictly _pari passu_; they
reach the same goal at the same time, but not by identical stages.
Thus: A is born the larger, then B overtakes and surpasses A, and is
in his turn overtaken by A, the end being that the twins, on
reaching adult life, are of the same size. This process would aid in
giving an interchangeable likeness at certain periods of their growth,
and is undoubtedly due to nature more frequently than to nurture.

Among my thirty-five detailed cases of close similarity, there are
no less than seven in which both twins suffered from some special
ailment or had some exceptional peculiarity. One twin writes that
she and her sister "have both the defect of not being able to come
downstairs quickly, which, however, was not born with them, but came
on at the age of twenty." Three pairs of twins have peculiarities in
their fingers; in one case it consists in a slight congenital
flexure of one of the joints of the little finger; it was inherited
from a grandmother, but neither parents, nor brothers, nor sisters
show the least trace of it. In another case the twins have a
peculiar way of bending the fingers, and there was a faint tendency
to the same peculiarity in the mother, but in her alone of all the
family. In a third case, about which I made a few inquiries, which
is given by Mr. Darwin, but is not included in my returns, there was
no known family tendency to the peculiarity which was observed in
the twins of having a crooked little finger. In another pair of twins,
one was born ruptured, and the other became so at six months old.
Two twins at the age of twenty-three were attacked by toothache, and
the same tooth had to be extracted in each case. There are curious
and close correspondences mentioned in the falling off of the hair.
Two cases are mentioned of death from the same disease; one of which
is very affecting. The outline of the story was that the twins were
closely alike and singularly attached, and had identical tastes;
they both obtain