level of the whole area in the course of six
million years. This seems an astonishing result, and some considerations
lead to the suspicion that it may be too large, but if halved or
quartered it is still very surprising. Few of us, however, know what a
million really means: Mr. Croll gives the following illustration: Take
a narrow strip of paper, eighty-three feet four inches in length, and
stretch it along the wall of a large hall; then mark off at one end
the tenth of an inch. This tenth of an inch will represent one hundred
years, and the entire strip a million years. But let it be borne in
mind, in relation to the subject of this work, what a hundred years
implies, represented as it is by a measure utterly insignificant in a
hall of the above dimensions. Several eminent breeders, during a single
lifetime, have so largely modified some of the higher animals, which
propagate their kind much more slowly than most of the lower animals,
that they have formed what well deserves to be called a new sub-breed.
Few men have attended with due care to any one strain for more than half
a century, so that a hundred years represents the work of two breeders
in succession. It is not to be supposed that species in a state of
nature ever change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer
with the effects which follow from unconscious selection, that is, the
preservation of the most useful or beautiful animals, with no intention
of modifying the breed; but by this process of unconscious selection,
various breeds have been sensibly changed in the course of two or three
centuries.

Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from
all the inhabitants of the same country being already so well adapted to
each other, that new places in the polity of nature do not occur until
after long intervals, due to the occurrence of physical changes of some
kind, or through the immigration of new forms. Moreover, variations
or individual differences of the right nature, by which some of the
inhabitants might be better fitted to their new places under the altered
circumstance, would not always occur at once. Unfortunately we have no
means of determining, according to the standard of years, how long a
period it takes to modify a species; but to the subject of time we must
return.

ON T