nature says the first and last
word. Francis Galton, the father of eugenics, hit on an ingenious and
more convincing illustration by studying the history of twins.[3]

There are, everyday observation shows, two kinds of twins--ordinary
twins and the so-called identical twins. Ordinary twins are merely
brothers, or sisters, or brother and sister, who happen to be born two
at a time, because two ova have developed simultaneously. The fact that
they were born at the same time does not make them alike--they differ
quite as widely from each other as ordinary brothers and sisters do.
Identical twins have their origin in a different phenomenon--they are
believed to be halves of the same egg-cell, in which two growing-points
appeared at a very early embryonic stage, each of these developing into
a separate individual. As would be expected, these identical twins are
always of the same sex, and extremely like each other, so that sometimes
their own mother can not tell them apart. This likeness extends to all
sorts of traits:--they have lost their milk teeth on the same day in one
case, they even fell ill on the same day with the same disease, even
though they were in different cities.

Now Galton reasoned that if environment really changes the inborn
character, then these identical twins, who start life as halves of the
same whole, ought to become more unlike if they were brought up apart;
and as they grew older and moved into different spheres of activity,
they ought to become measurably dissimilar. On the other hand, ordinary
twins, who start dissimilar, ought to become more alike when brought up
in the same family, on the same diet, among the same friends, with the
same education. If the course of years shows that identical twins remain
as like as ever and ordinary twins as unlike as ever, regardless of
changes in conditions, then environment will have failed to demonstrate
that it has any great power to modify one's inborn nature in these
traits.

With this view, Galton collected the history of eighty pairs of
identical twins, thirty-five cases being accompanied by very full
details, which showed that the twins were really as nearly identical, in
childhood, as one could expect to find. On this point, Galton's
inquiries were careful, and the replies satisfactory. They are not,
however, as he remarks, much varied in character. "When the twins are
children, they are usually distinguished by ribbons tied around the
wrist or n