On an ordinary big clock we do not readily see that even the minute hand
is moving, and if the clock struck only once in a hundred years we can
conceive of people arguing whether the hands did really move at all. So
it often is with the changes that go on from generation to generation in
living creatures. The flux is so slow, like the flowing of a glacier,
that some people fail to be convinced of its reality. And it must, of
course, be admitted that some kinds of living creatures, like the
Lamp-shell _Ligula_ or the Pearly Nautilus, hardly change from age to
age, whereas others, like some of the birds and butterflies, are always
giving rise to something new. The Evening Primrose among plants, and the
Fruit-fly, Drosophila, among animals, are well-known examples of
organisms which are at present in a sporting or mutating mood.
Certain dark varieties of moth, e.g. of the Peppered Moth, are taking
the place of the paler type in some parts of England, and the same is
true of some dark forms of Sugar-bird in the West Indian islands. Very
important is the piece of statistics worked out by Professor R. C.
Punnett, that "if a population contains .001 per cent of a new variety,
and if that variety has even a 5 per cent selection advantage over the
original form, the latter will almost completely disappear in less than
a hundred generations." This sort of thing has been going on all over
the world for untold ages, and the face of animate nature has
consequently changed.
We are impressed by striking novelties that crop up--a clever dwarf, a
musical genius, a calculating boy, a cock with a 10 ft. tail, a
"wonder-horse" with a mane reaching to the ground, a tailless cat, a
white blackbird, a copper beech, a Greater Celandine with much cut up
leaves; but this sort of mutation is common, and smaller, less brusque
variations are commoner still. _They form the raw materials of possible
evolution._ We are actually standing before an apparently inexhaustible
fountain of change. This is evolution going on.