ery 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