Applied Eugenics | Page 4

brought to

bear after birth, even though these latter influences include such
powerful ones as nutrition and education within ordinary limits.
But the biological evidence does not lend itself readily to summary
treatment, and we shall therefore examine the question by statistical
methods.[2] These have the further advantage of being more easily
understood; for facts which can be measured and expressed in numbers
are facts whose import the reader can usually decide for himself: he is
perfectly able to determine, without any special training, whether twice
two does or does not make four. One further preliminary remark: the
problem of nature vs. nurture can not be solved in general terms; a
moment's thought will show that it can be understood only by
examining one trait at a time. The problem is to decide whether the
differences between the people met in everyday life are due more to
inheritance or to outside influences, and these differences must
naturally be examined separately; they can not be lumped together.
To ask whether nature in general contributes more to a man than
nurture is futile; but it is not at all futile to ask whether the differences
in a given human trait are more affected by differences in nature than
by differences in nurture. It is easy to see that a verdict may be
sometimes given to one side, sometimes to the other. Albinism in
animals, for instance, is a trait which is known to be inherited, and
which is very slightly affected by differences of climate, food supply,
etc. On the other hand, there are factors which, although having
inherited bases, owe their expression almost wholly to outside
influences. Professor Morgan, for example, has found a strain of fruit
flies whose offspring in cold weather are usually born with
supernumerary legs. In hot weather they are practically normal. If this
strain were bred only in the tropics, the abnormality would probably
not be noticed; on the other hand, if it were bred only in cold regions, it
would be set down as one characterized by duplication of limbs. The
heredity factor would be the same in each case, the difference in
appearance being due merely to temperature.
Mere inspection does not always tell whether some feature of an
individual is more affected by changes in heredity or changes in

surroundings. On seeing a swarthy man, one may suppose that he
comes of a swarthy race, or that he is a fair-skinned man who has lived
long in the desert. In the one case the swarthiness would be inheritable,
in the other not. Which explanation is correct, can only be told by
examining a number of such individuals under critical conditions, or by
an examination of the ancestry. A man from a dark-skinned race would
become little darker by living under the desert sun, while a white man
would take on a good deal of tan.
The limited effect of nurture in changing nature is in some fields a
matter of common observation. The man who works in the gymnasium
knows that exercise increases the strength of a given group of muscles
for a while, but not indefinitely. There comes a time when the limit of a
man's hereditary potentiality is reached, and no amount of exercise will
add another millimeter to the circumference of his arm. Similarly the
handball or tennis player some day reaches his highest point, as do
runners or race horses. A trainer could bring Arthur Duffy in a few
years to the point of running a hundred yards in 9-3/5 seconds, but no
amount of training after that could clip off another fifth of a second. A
parallel case is found in the students who take a college examination.
Half a dozen of them may have devoted the same amount of time to
it--may have crammed to the limit--but they will still receive widely
different marks. These commonplace cases show that nurture has
seemingly some power to mold the individual, by giving his inborn
possibilities a chance to express themselves, but that 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