onvincing. In the last paragraph but one, we mentioned
Galton's study (cited at some length in our Chapter I) of "identical"
twins, who are so much alike at birth for the very good reason that they
have identical heredity. This heredity was found to be not modified,
either in the body or the mind, by ordinary differences of training and
environment. Some of Galton's histories[35] of ordinary, non-identical
twins were also given in Chapter I; two more follow:

One parent says: "They have been treated exactly alike; both were
brought up by hand; they have been under the same nurse and governess
from their birth, and they are very fond of each other. Their increasing
dissimilarity must be ascribed to a natural difference of mind and
character, as there has been nothing in their treatment to account for
it."

Another writes: "This case is, I should think, somewhat remarkable for
dissimilarity in physique as well as for strong contrast in character.
They have been unlike in mind and body throughout their lives. Both were
reared in a country house and both were at the same schools until the
age of 16."

In the face of such examples, can anyone maintain that differences in
mental make-up are wholly due to different influences during childhood,
and not at all to differences in germinal make-up? It is not necessary
to depend, under this head, on mere descriptions, for accurate
measurements are available to demonstrate the point. If the environment
creates the mental nature, then ordinary brothers, not more than four or
five years apart in age, ought to be about as closely similar to each
other as identical twins are to each other; for the family influences in
each case are practically the same. Professor Thorndike, by careful
mental tests, showed[36] that this is not true. The ordinary brothers
come from different egg-cells, and, as is known from studies on lower
animals, they do not get exactly the same inheritance from their
parents; they show, therefore, considerable differences in their psychic
natures. Real identical twins, being two halves of the same egg-cell,
have the same heredity, and their natures are therefore much more
nearly identical.

Again, if the mind is molded during the "plastic years of childhood,"
children ought to become more alike, the longer they are together. Twins
who were unlike at birth ought to resemble each other more closely at 14
than they did at 9, since they have been for five additional ye