in mind that some are extremely ancient, and must have branched off
from a common parent at a remote epoch; so that in such cases there
will have been ample time for great climatal and geographical changes
and for accidents of transport; and consequently for the migration of
some of the species into all
quarters of the world, where they may have become slightly modified in
relation to their new conditions. There is, also, some reason to
believe from geological evidence that organisms low in the scale within
each great class, generally change at a slower rate than the higher
forms; and consequently the lower forms will have had a better chance
of ranging widely and of still retaining the same specific character.
This fact, together with the seeds and eggs of many low forms being
very minute and better fitted for distant transportation, probably
accounts for a law which has long been observed, and which has lately
been admirably discussed by Alph. de Candolle in regard to plants,
namely, that the lower any group of organisms is, the more widely it is
apt to range.
The relations just discussed, namely, low and slowly-changing organisms
ranging more widely than the high, some of the species of
widely-ranging genera themselves ranging widely, such facts, as alpine,
lacustrine, and marsh productions being related (with the exceptions
before specified) to those on the surrounding low lands and dry lands,
though these stations are so different the very close relation of the
distinct species which inhabit the islets of the same archipelago, and
especially the striking relation of the inhabitants of each whole
archipelago or island to those of the nearest mainland, are, I think,
utterly inexplicable on the ordinary view of the independent creation
of each species, but are explicable on the view of colonisation from
the nearest and readiest source, together with the subsequent
modification and better adaptation of the colonists to their new homes.
_Summary of last and present Chapters_. In these chapters I have
endeavoured to show, that if we make due allowance for our ignorance of
the full effects of all
the changes of climate and of the level of the land, which have
certainly occurred within the recent period, and of other similar
changes which may have occurred within the same period; if we remember
how profoundly ignorant we are with respect to the many and curious
means of occasional transport, a subject which has hardly ever been
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