There is also reason to believe, from
geological evidence, that within each great class the lower organisms
change at a slower rate than the higher; consequently they will have had a
better chance of ranging widely and of still retaining the same specific
character. This fact, together with that of the seeds and eggs of most
lowly organised forms being very minute and better fitted for distant
transportal, probably accounts for a law which has long been observed, and
which has lately been discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms stands the more widely it
ranges.
The relations just discussed--namely, lower organisms ranging more widely
than the higher--some of the species of widely-ranging genera themselves
ranging widely--such facts, as alpine, lacustrine, and marsh productions
being generally related to those which live on the surrounding low lands
and dry lands--the striking relationship between the inhabitants of islands
and those of the nearest mainland--the still closer relationship of the
distinct inhabitants of the islands of the same archipelago--are
inexplicable on the ordinary view of the independent creation of each
species, but are explicable if we admit colonisation from the nearest or
readiest source, together with the subsequent adaptation of the colonists
to their new homes.
SUMMARY OF THE LAST AND PRESENT CHAPTERS.
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