CHAPTER X. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
2. ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF DEPOSITION AND EXTENT OF DENUDATION. (continued)
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.