The Editor: Heredity and the Mind 
preliminary remark about the nature 
of twins. 
There appear to be two ways in which 
twins are ordinarily produced. They 
may be the result of the simultaneous 
fertilization of two egg-cells, in which 
case they are no different from ordinary 
brothers, or sisters, except that they 
happen to be born simultaneously. On 
the other hand, they may be produced 
by a division of a single egg-cell, at an 
early stage in development; in such 
cases they are always of the same sex, 
and very closely alike, as one would 
expect from the fact that they are really 
halves of the same individual. 
In the former quotation from Galton, 
we dealt with the second class, the so- 
called identical twins, who are very 
much alike at birth for the good reason 
that they have identical heredity. We 
found that this heredity was not modi- 
fied, either in the body or in the mind, 
by ordinary differences of training and 
environment. A few of Galton’s his- 
tories of ordinary, non-identical twins, 
follow: 
One parent says: “They have had 
exactly the same nurture from their birth 
up to the present time; they are both 
perfectly healthy and strong, yet they are 
otherwise as dissimilar as two boys could 
be, physically, mentally, and in their 
emotional nature.”’ 
Another writes: “I can answer most 
decidedly that the twins have been 
perfectly dissimilar in character, habits 
and likeness from the moment of. their 
birth to the present time, although they 
were nursed by the same woman, went to 
school together, and were never separated 
until the age of 15.” 
“Very dissimilar in body and mind,”’ is 
the description of another parent. ‘‘The 
one is quiet, retiring, and slow but sure; 
good tempered, but disposed to be sulky 
when provoked—the other is quick, 
vivacious, forward, acquiring easily and 
forgetting soon; quick-tempered and chol- 
eric but easily forgetting and forgiving. 
They have been educated together and 
never separated.”’ 
Again, ‘“The two sisters are very differ- 
ent in ability and disposition. The one is 
retiring, but firm and determined; she has 
no taste for music or drawing. The other 
459 
is of an active, excitable temperament: 
she displays an unusual amount of quick- 
ness and talent, and is passionately fond 
of music and drawing. From infancy, 
they have rarely been separated even at 
school, and as children visiting their 
friends, they always were together.”’ 
If, in the face of such examples, the 
psychologist can maintain that differ- 
ences in mental make-up are due to 
different influences during childhood, 
and not to differences in heredity, he 
certainly has a colossal faith in his 
theories. We are not obliged to depend, 
under this head, for mere descriptions, 
but can supply accurate measurements 
to. ‘demonstrate our pointy site waue 
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. Thorn- 
dike, by careful mental tests, showed? 
that this is not true. The ordinary 
brothers come from different egg-cells, 
and, as we know from studies on lower 
animals, they do not get exactly the 
same inheritance from their parents; 
they show, therefore, considerable differ- 
ences in their psychic natures. Real 
identical twins are two halves of the 
same egg-cell, they halve the same 
heredity, and their natures are therefore 
much more nearly identical. 
Again, if the mind is molded during 
the ‘“‘plastic years of childhood,” chil- 
dren 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 years subjected to this 
supposedly potent but very mystical 
“molding force.” Here again Thorn- 
dike’s exact measurements explode the 
fallacy. They are actually, measurably, 
less alike at the older age; their inborn 
natures are developing along predestined 
lines, with little regard to the identity 
9 Thorndike, E. L. Measurements of Twins. Arch. of Philos., Psych., and Sci. Methods, 
No. 1, New York, 1905; summarized in his Educational Psychology, Vol. III, pp. 247—251,New 
York, 1914. Measured on a scale where 1=identity, he found that twins showed a resemblance 
to each other of about .75, while ordinary brothers of about the same age resembled each other to 
the extent of about .50 only. The resemblance was approximately the same in both physical and 
mental traits. 
