Intercrossing
with the inhabitants of the surrounding districts, will also be thus
prevented. Moritz Wagner has lately published an interesting essay on this
subject, and has shown that the service rendered by isolation in preventing
crosses between newly-formed varieties is probably greater even than I
supposed. But from reasons already assigned I can by no means agree with
this naturalist, that migration and isolation are necessary elements for
the formation of new species. The importance of isolation is likewise
great in preventing, after any physical change in the conditions, such as
of climate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district will
be left open to be filled up by the modification of the old inhabitants.
Lastly, isolation will give time for a new variety to be improved at a slow
rate; and this may sometimes be of much importance. If, however, an
isolated area be very small, either from being surrounded by barriers, or
from having very peculiar physical conditions, the total number of the
inhabitants will be small; and this will retard the production of new
species through natural selection, by decreasing the chances of favourable
variations arising.
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law.
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