had
previously been able to carry out--and I have tried many methods--is
wholly free from this objection. I have therefore attacked the
problem from the opposite side, seeking for some new method by which
it would be possible to weigh in just scales the effects of Nature
and Nurture, and to ascertain their respective shares in framing the
disposition and intellectual ability of men. The life-history of
twins supplies what I wanted. We may begin by inquiring about twins
who were closely alike in boyhood and youth, and who were educated
together for many years, and learn whether they subsequently grew
unlike, and, if so, what the main causes were which, in the opinion
of the family, produced the dissimilarity. In this way we can obtain
direct evidence of the kind we want. Again, we may obtain yet more
valuable evidence by a converse method. We can inquire into the
history of twins who were exceedingly unlike in childhood, and learn
how far their characters became assimilated under the influence of
identical nurture, inasmuch as they had the same home, the same
teachers, the same associates, and in every other respect the same
surroundings.

My materials were obtained by sending circulars of inquiry to
persons who were either twins themselves or near relations of twins.
The printed questions were in thirteen groups; the last of them
asked for the addresses of other twins known to the recipient, who
might be likely to respond if I wrote to them. This happily led to a
continually widening circle of correspondence, which I pursued until
enough material was accumulated for a general reconnaisance of the
subject.

There is a large literature relating to twins in their purely
surgical and physiological aspect. The reader interested in this
should consult _Die Lehre von den Zwillingen_, von L. Kleinwaechter,
Prag. 1871. It is full of references, but it is also unhappily
disfigured by a number of numerical misprints, especially in page 26.
I have not found any book that treats of twins from my present point
of view.

The reader will easily understand that the word "twins" is a vague
expression, which covers two very dissimilar events--the one
corresponding to the progeny of animals that usually bear more than
one at a birth, each of the progeny being derived from a separate
ovum, while the other event is due to the development of two
germinal spots in the same ovum. In the latter case they are
enveloped in the same membrane