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.
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